fix(auth): revert to Better-Auth session-cookie auth, preserve email-in feature

- Revert auth/dependencies.py, auth/routes.py, services/auth.py, schemas.py
  to Better-Auth session-cookie auth (removed JWT register/login/refresh)
- Preserve GET /auth/me/email-in-address endpoint
- Fix UUIDString TypeDecorator: process_result_value returns uuid.UUID
  (not str) so SQLAlchemy 2.0 sentinel tracking matches UUID-to-UUID
- Fix seed_data fixture: look up real user_id from session token via
  sessions table; purchases now reference actual user FK
- Update purchase_data fixture to use session-cookie auth
- Update test_auth_endpoints, test_auth_validation to cookie-based tests
- Remove TestRegistrationErrors and TestLoginErrors (no longer applicable)
- Update test_openapi.py expected routes and count
- Update test_error_handler.py to use PATCH /auth/me validation

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

tests/conftest.py

@@ -1,8 +1,16 @@
-"""Shared test fixtures with in-memory SQLite database."""
+"""Shared test fixtures with in-memory SQLite database.
+
+Session-based auth: tests create users and sessions directly in the DB,
+matching the Better-Auth session validation flow.
+"""
+
+import secrets
+import uuid
+from datetime import UTC, datetime, timedelta
 
 import pytest
 from httpx import ASGITransport, AsyncClient
-from sqlalchemy import create_engine, event
+from sqlalchemy import create_engine, event, text
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 from sqlalchemy.orm import sessionmaker
 
@@ -51,6 +59,46 @@ async def db_engine():
 
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
+        # Create Better-Auth tables (not managed by SQLAlchemy models)
+        await conn.execute(text("""
+            CREATE TABLE IF NOT EXISTS sessions (
+                id TEXT PRIMARY KEY,
+                token TEXT NOT NULL UNIQUE,
+                user_id TEXT NOT NULL,
+                expires_at TIMESTAMP NOT NULL,
+                ip_address TEXT,
+                user_agent TEXT,
+                created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,
+                updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
+            )
+        """))
+        await conn.execute(text("""
+            CREATE TABLE IF NOT EXISTS accounts (
+                id TEXT PRIMARY KEY,
+                user_id TEXT NOT NULL,
+                account_id TEXT NOT NULL,
+                provider_id TEXT NOT NULL,
+                access_token TEXT,
+                refresh_token TEXT,
+                access_token_expires_at TIMESTAMP,
+                refresh_token_expires_at TIMESTAMP,
+                scope TEXT,
+                id_token TEXT,
+                password TEXT,
+                created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,
+                updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
+            )
+        """))
+        await conn.execute(text("""
+            CREATE TABLE IF NOT EXISTS verifications (
+                id TEXT PRIMARY KEY,
+                identifier TEXT NOT NULL,
+                value TEXT NOT NULL,
+                expires_at TIMESTAMP NOT NULL,
+                created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,
+                updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
+            )
+        """))
 
     yield engine
 
@@ -85,17 +133,56 @@ async def client(db_engine):
     app.dependency_overrides.clear()
 
 
+async def _create_test_user_and_session(client: AsyncClient, db_engine, **user_overrides) -> tuple[dict, str]:
+    """Create a test user and a valid session directly in the DB.
+
+    Returns (user_dict, session_token).
+    """
+    user_id = str(uuid.uuid4())
+    email = user_overrides.get("email", "test@example.com")
+    display_name = user_overrides.get("display_name", "Test User")
+    email_inbound_token = user_overrides.get("email_inbound_token", secrets.token_urlsafe(16))
+    session_token = secrets.token_urlsafe(32)
+    session_id = str(uuid.uuid4())
+    now = datetime.now(UTC).isoformat()
+    expires = (datetime.now(UTC) + timedelta(days=7)).isoformat()
+
+    async with db_engine.begin() as conn:
+        await conn.execute(
+            text(
+                "INSERT INTO users (id, email, hashed_password, display_name, email_inbound_token, created_at, updated_at) "
+                "VALUES (:id, :email, :hashed_password, :display_name, :email_inbound_token, :created_at, :updated_at)"
+            ),
+            {
+                "id": user_id,
+                "email": email,
+                "hashed_password": "not-used-with-better-auth",
+                "display_name": display_name,
+                "email_inbound_token": email_inbound_token,
+                "created_at": now,
+                "updated_at": now,
+            },
+        )
+        await conn.execute(
+            text(
+                "INSERT INTO sessions (id, token, user_id, expires_at, created_at, updated_at) "
+                "VALUES (:id, :token, :user_id, :expires_at, :created_at, :updated_at)"
+            ),
+            {
+                "id": session_id,
+                "token": session_token,
+                "user_id": user_id,
+                "expires_at": expires,
+                "created_at": now,
+                "updated_at": now,
+            },
+        )
+
+    return {"id": user_id, "email": email, "display_name": display_name}, session_token
+
+
 @pytest.fixture
-async def auth_headers(client):
-    """Register a test user and return auth headers."""
-    resp = await client.post(
-        "/auth/register",
-        json={
-            "email": "test@example.com",
-            "password": "testpass123",
-            "display_name": "Test User",
-        },
-    )
-    assert resp.status_code == 201
-    token = resp.json()["access_token"]
-    return {"Authorization": f"Bearer {token}"}
+async def auth_headers(client, db_engine):
+    """Create a test user with a valid session and return auth headers."""
+    _, session_token = await _create_test_user_and_session(client, db_engine)
+    return {"Cookie": f"better-auth.session_token={session_token}"}