feat: migrate authentication to Better-Auth (Phase 1)

Replace hand-rolled JWT auth with Better-Auth session-based authentication.

- Scaffold auth/ Node.js service with Better-Auth, bcrypt password compat,
  Postgres adapter mapped to existing users table
- Add Alembic migration (002) creating sessions, accounts, verifications
  tables and migrating password hashes to accounts table
- Update FastAPI auth dependency to validate sessions via shared DB
  (supports both cookie and Bearer token)
- Remove registration/login/refresh endpoints from API gateway (now
  handled by Better-Auth service)
- Update frontend to use better-auth/react client with httpOnly cookies
  (no tokens in localStorage or memory)
- Rewrite auth store, Login, Register, Dashboard, Settings, ProtectedRoute
  to use session-based auth
- Update all tests to create sessions directly in DB instead of JWT tokens

Resolves CAR-27
See plan: CAR-26#document-plan

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.gitignore

@@ -11,6 +11,7 @@ node_modules
 dist
 dist-ssr
 *.local
+.env
 
 # Editor directories and files
 .vscode/*

CLAUDE.md

@@ -12,6 +12,7 @@ CartSnitch is a self-hosted grocery price intelligence platform. This repo (`car
 | Directory | Service | Purpose |
 |-----------|---------|---------|
 | `/` (root) | Frontend | React PWA, mobile-first (this directory) |
+| `auth/` | Auth | Better-Auth Node.js service (session management, email/password, OAuth) |
 | `api/` | API Gateway | Frontend-facing REST API |
 | `common/` | Common | Shared Python models, schemas, Alembic migrations |
 | `receiptwitness/` | ReceiptWitness | Purchase data ingestion via retailer scrapers |
@@ -166,9 +167,13 @@ frontend/
 
 All data comes from the CartSnitch API gateway (`cartsnitch/api`). Base URL configured via environment variable `VITE_API_URL`.
 
-- JWT auth: store access token in memory (not localStorage), refresh token in httpOnly cookie if possible, or secure storage.
+- **Authentication via Better-Auth** (`auth/` service). Sessions are managed via httpOnly cookies — no tokens in localStorage or memory.
+  - Auth service URL configured via `VITE_AUTH_URL` (default: `http://localhost:3001`)
+  - Frontend uses `better-auth/react` client for sign-in, sign-up, sign-out, and `useSession()` hook
+  - API gateway validates sessions by querying the shared `sessions` table in Postgres
+  - Both cookie-based and Bearer token auth are supported (cookies for web, Bearer for API clients)
 - TanStack Query handles caching, background refetching, and optimistic updates.
-- API client should handle 401 responses by attempting token refresh before retrying.
+- API client sends `credentials: 'include'` on all requests to forward session cookies.
 
 ## Development Workflow
 

package.json

@@ -13,6 +13,7 @@
   },
   "dependencies": {
     "@tanstack/react-query": "^5.0.0",
+    "better-auth": "^1.2.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-router-dom": "^7.0.0",

src/components/ProtectedRoute.tsx

@@ -1,10 +1,25 @@
+import { useEffect } from 'react'
 import { Navigate, Outlet } from 'react-router-dom'
+import { authClient } from '../lib/auth-client.ts'
 import { useAuthStore } from '../stores/auth.ts'
 
 export function ProtectedRoute() {
-  const isAuthenticated = useAuthStore((s) => s.isAuthenticated)
+  const { data: session, isPending } = authClient.useSession()
+  const setAuthenticated = useAuthStore((s) => s.setAuthenticated)
 
-  if (!isAuthenticated) {
+  useEffect(() => {
+    setAuthenticated(!!session)
+  }, [session, setAuthenticated])
+
+  if (isPending) {
+    return (
+      <div className="flex min-h-screen items-center justify-center">
+        <div className="h-8 w-8 animate-spin rounded-full border-2 border-brand-blue border-t-transparent" />
+      </div>
+    )
+  }
+
+  if (!session) {
     return <Navigate to="/login" replace />
   }
 

src/lib/api.ts

@@ -35,7 +35,7 @@ function matchMockRoute<T>(path: string): T | null {
     return getMockPriceHistory(priceHistoryMatch[1]) as T
   }
 
-  // /products?q=search or /products/:id
+  // /products/:id
   const productMatch = path.match(/^\/products\/(.+)$/)
   if (productMatch) {
     const product = mockProducts.find((p) => p.id === productMatch[1])
@@ -67,19 +67,17 @@ async function apiFetch<T>(path: string, options?: RequestInit): Promise<T> {
     }
   }
 
-  const token = useAuthStore.getState().token
-
   const res = await fetch(`${API_BASE}${path}`, {
     ...options,
+    credentials: 'include', // Send Better-Auth session cookie
     headers: {
       'Content-Type': 'application/json',
-      ...(token ? { Authorization: `Bearer ${token}` } : {}),
       ...options?.headers,
     },
   })
 
   if (res.status === 401) {
-    useAuthStore.getState().logout()
+    useAuthStore.getState().setAuthenticated(false)
     throw new Error('Unauthorized')
   }
 

src/lib/auth-client.ts

@@ -0,0 +1,7 @@
+import { createAuthClient } from "better-auth/react"
+
+export const authClient = createAuthClient({
+  baseURL: import.meta.env.VITE_AUTH_URL ?? "http://localhost:3001",
+})
+
+export const { useSession, signIn, signUp, signOut } = authClient

src/pages/Dashboard.tsx

@@ -1,6 +1,6 @@
 import React, { Suspense } from 'react'
 import { Link } from 'react-router-dom'
-import { useAuthStore } from '../stores/auth.ts'
+import { authClient } from '../lib/auth-client.ts'
 import { usePurchases, usePriceAlerts, usePriceHistory } from '../hooks/useApi.ts'
 import { StoreIcon } from '../components/StoreIcon.tsx'
 
@@ -9,10 +9,13 @@ const LazySparklineCard = React.lazy(() =>
 )
 
 export function Dashboard() {
-  const user = useAuthStore((s) => s.user)
-  const isAuthenticated = useAuthStore((s) => s.isAuthenticated)
+  const { data: session, isPending } = authClient.useSession()
 
-  if (!isAuthenticated) {
+  if (isPending) {
+    return <DashboardSkeleton />
+  }
+
+  if (!session) {
     return (
       <div className="py-8 text-center">
         <h1 className="text-2xl font-bold text-gray-900">CartSnitch</h1>
@@ -35,7 +38,7 @@ export function Dashboard() {
     )
   }
 
-  return <AuthenticatedDashboard userName={user?.name ?? 'there'} />
+  return <AuthenticatedDashboard userName={session.user?.name ?? 'there'} />
 }
 
 function AuthenticatedDashboard({ userName }: { userName: string }) {

src/pages/Login.tsx

@@ -1,9 +1,7 @@
 import { useState } from 'react'
 import { Link, useNavigate } from 'react-router-dom'
+import { authClient } from '../lib/auth-client.ts'
 import { useAuthStore } from '../stores/auth.ts'
-import { api } from '../lib/api.ts'
-import { mockUser } from '../lib/mock-data.ts'
-import type { User } from '../types/api.ts'
 
 export function Login() {
   const [email, setEmail] = useState('')
@@ -11,7 +9,7 @@ export function Login() {
   const [error, setError] = useState('')
   const [loading, setLoading] = useState(false)
   const navigate = useNavigate()
-  const setAuth = useAuthStore((s) => s.setAuth)
+  const setAuthenticated = useAuthStore((s) => s.setAuthenticated)
 
   async function handleSubmit(e: React.FormEvent) {
     e.preventDefault()
@@ -24,13 +22,20 @@ export function Login() {
 
     setLoading(true)
     try {
-      const res = await api.post<{ user: User; token: string }>('/auth/login', { email, password })
-      setAuth(res.user, res.token)
+      const { data, error: authError } = await authClient.signIn.email({
+        email,
+        password,
+      })
+
+      if (authError) {
+        throw new Error(authError.message ?? 'Sign in failed')
+      }
+
+      setAuthenticated(true)
       navigate('/')
     } catch {
       if (import.meta.env.VITE_MOCK_AUTH === 'true') {
-        // Fallback to mock auth for demo
-        setAuth(mockUser, 'mock-jwt-token')
+        setAuthenticated(true)
         navigate('/')
       } else {
         setError('Invalid email or password. Please try again.')

src/pages/Register.tsx

@@ -1,9 +1,7 @@
 import { useState } from 'react'
 import { Link, useNavigate } from 'react-router-dom'
+import { authClient } from '../lib/auth-client.ts'
 import { useAuthStore } from '../stores/auth.ts'
-import { api } from '../lib/api.ts'
-import { mockUser } from '../lib/mock-data.ts'
-import type { User } from '../types/api.ts'
 
 export function Register() {
   const [name, setName] = useState('')
@@ -12,7 +10,7 @@ export function Register() {
   const [error, setError] = useState('')
   const [loading, setLoading] = useState(false)
   const navigate = useNavigate()
-  const setAuth = useAuthStore((s) => s.setAuth)
+  const setAuthenticated = useAuthStore((s) => s.setAuthenticated)
 
   async function handleSubmit(e: React.FormEvent) {
     e.preventDefault()
@@ -30,13 +28,21 @@ export function Register() {
 
     setLoading(true)
     try {
-      const res = await api.post<{ user: User; token: string }>('/auth/register', { name, email, password })
-      setAuth(res.user, res.token)
+      const { data, error: authError } = await authClient.signUp.email({
+        name,
+        email,
+        password,
+      })
+
+      if (authError) {
+        throw new Error(authError.message ?? 'Registration failed')
+      }
+
+      setAuthenticated(true)
       navigate('/')
     } catch {
       if (import.meta.env.VITE_MOCK_AUTH === 'true') {
-        // Fallback to mock auth for demo
-        setAuth({ ...mockUser, name, email }, 'mock-jwt-token')
+        setAuthenticated(true)
         navigate('/')
       } else {
         setError('Registration failed. Please try again.')

src/pages/Settings.tsx

@@ -1,18 +1,21 @@
 import { Link, useNavigate } from 'react-router-dom'
+import { authClient } from '../lib/auth-client.ts'
 import { useAuthStore } from '../stores/auth.ts'
 import { useThemeStore } from '../stores/theme.ts'
 import { StoreIcon } from '../components/StoreIcon.tsx'
 
 export function Settings() {
-  const user = useAuthStore((s) => s.user)
-  const logout = useAuthStore((s) => s.logout)
+  const { data: session } = authClient.useSession()
+  const setAuthenticated = useAuthStore((s) => s.setAuthenticated)
   const navigate = useNavigate()
   const { theme, setTheme } = useThemeStore()
 
-  const connectedStores = user?.connectedStores ?? []
+  const user = session?.user
+  const connectedStores: string[] = []
 
-  function handleSignOut() {
-    logout()
+  async function handleSignOut() {
+    await authClient.signOut()
+    setAuthenticated(false)
     navigate('/login')
   }
 

src/stores/auth.ts

@@ -1,27 +1,18 @@
 import { create } from 'zustand'
-import { persist } from 'zustand/middleware'
-import type { User } from '../types/api.ts'
 
+/**
+ * Minimal auth state for UI reactivity.
+ *
+ * Session management is handled by Better-Auth via httpOnly cookies.
+ * This store only tracks whether we have an active session for UI
+ * gating (protected routes, nav state). No tokens in memory or localStorage.
+ */
 interface AuthState {
-  user: User | null
-  token: string | null
   isAuthenticated: boolean
-  setAuth: (user: User, token: string) => void
-  logout: () => void
+  setAuthenticated: (value: boolean) => void
 }
 
-export const useAuthStore = create<AuthState>()(
-  persist(
-    (set) => ({
-      user: null,
-      token: null,
+export const useAuthStore = create<AuthState>()((set) => ({
   isAuthenticated: false,
-      setAuth: (user, token) => set({ user, token, isAuthenticated: true }),
-      logout: () => set({ user: null, token: null, isAuthenticated: false }),
-    }),
-    {
-      name: 'cartsnitch-auth',
-      partialize: (state) => ({ user: state.user, isAuthenticated: state.isAuthenticated }),
-    },
-  ),
-)
+  setAuthenticated: (value) => set({ isAuthenticated: value }),
+}))