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>

alembic/versions/002_better_auth_tables.py

@@ -0,0 +1,101 @@
+"""Add Better-Auth tables and extend users table.
+
+Creates sessions, accounts, and verifications tables for Better-Auth.
+Adds email_verified and image columns to existing users table.
+Migrates password hashes from users.hashed_password to accounts.password.
+
+Revision ID: 002_better_auth_tables
+Revises: 001_encrypt_session_data
+Create Date: 2026-03-28
+"""
+
+import sqlalchemy as sa
+from sqlalchemy import text
+
+from alembic import op
+
+revision = "002_better_auth_tables"
+down_revision = "001_encrypt_session_data"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # --- Extend users table for Better-Auth compatibility ---
+    op.add_column("users", sa.Column("email_verified", sa.Boolean(), nullable=False, server_default="false"))
+    op.add_column("users", sa.Column("image", sa.Text(), nullable=True))
+
+    # --- Create sessions table ---
+    op.create_table(
+        "sessions",
+        sa.Column("id", sa.Text(), nullable=False),
+        sa.Column("token", sa.Text(), nullable=False),
+        sa.Column("user_id", sa.Text(), nullable=False),
+        sa.Column("expires_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("ip_address", sa.Text(), nullable=True),
+        sa.Column("user_agent", sa.Text(), nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index("ix_sessions_token", "sessions", ["token"], unique=True)
+    op.create_index("ix_sessions_user_id", "sessions", ["user_id"])
+
+    # --- Create accounts table ---
+    op.create_table(
+        "accounts",
+        sa.Column("id", sa.Text(), nullable=False),
+        sa.Column("user_id", sa.Text(), nullable=False),
+        sa.Column("account_id", sa.Text(), nullable=False),
+        sa.Column("provider_id", sa.Text(), nullable=False),
+        sa.Column("access_token", sa.Text(), nullable=True),
+        sa.Column("refresh_token", sa.Text(), nullable=True),
+        sa.Column("access_token_expires_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("refresh_token_expires_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("scope", sa.Text(), nullable=True),
+        sa.Column("id_token", sa.Text(), nullable=True),
+        sa.Column("password", sa.Text(), nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index("ix_accounts_user_id", "accounts", ["user_id"])
+
+    # --- Create verifications table ---
+    op.create_table(
+        "verifications",
+        sa.Column("id", sa.Text(), nullable=False),
+        sa.Column("identifier", sa.Text(), nullable=False),
+        sa.Column("value", sa.Text(), nullable=False),
+        sa.Column("expires_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+    )
+
+    # --- Migrate existing password hashes to accounts table ---
+    # For each user with a hashed_password, create a 'credential' account row
+    conn = op.get_bind()
+    users = conn.execute(
+        text("SELECT id, hashed_password FROM users WHERE hashed_password IS NOT NULL")
+    ).fetchall()
+
+    for user_id, hashed_password in users:
+        user_id_str = str(user_id)
+        conn.execute(
+            text(
+                "INSERT INTO accounts (id, user_id, account_id, provider_id, password, created_at, updated_at) "
+                "VALUES (gen_random_uuid()::text, :user_id, :account_id, 'credential', :password, now(), now())"
+            ),
+            {"user_id": user_id_str, "account_id": user_id_str, "password": hashed_password},
+        )
+
+
+def downgrade() -> None:
+    op.drop_table("verifications")
+    op.drop_table("accounts")
+    op.drop_index("ix_sessions_user_id", table_name="sessions")
+    op.drop_index("ix_sessions_token", table_name="sessions")
+    op.drop_table("sessions")
+    op.drop_column("users", "image")
+    op.drop_column("users", "email_verified")