Migrate authentication to Better Auth

Replace the hand-rolled JWT auth with Better Auth 1.6.25 httpOnly cookie
sessions, validated against the database on every request so revocation,
bans and role changes take effect immediately.

Backend:
- betterAuth.ts wires the Drizzle adapter, magic links, Google sign-in and
  the admin plugin; auth-schema.ts maps Better Auth's models onto the
  existing `users` table so user IDs and their foreign keys survive intact.
- routes/auth.ts is gone; Better Auth serves the standard endpoints and
  authExt.ts carries the flows it doesn't cover.
- auth.ts shrinks to session resolution and helpers; sessions/revocation in
  dashboard.ts now read and delete `auth_sessions` rows directly.
- Schema adds the Better Auth core + admin columns (email_verified, image,
  banned, ban_reason, ban_expires), with migrations and tests.
- rateLimit.ts resolves client IPs spoof-resistantly: proxy headers are only
  honoured from loopback/RFC1918 peers plus TRUSTED_PROXIES.
- passwordPolicy.ts centralises password validation.
- Bump drizzle-orm, drizzle-kit and better-sqlite3 to versions compatible
  with Better Auth.

Frontend:
- auth-client.ts plus a reworked AuthContext and api/client.ts move to
  cookie-based sessions; no more bearer tokens in requests or middleware.

photo-api:
- Validate Better Auth session cookies against the shared auth_sessions
  table instead of verifying JWTs; JWT_SECRET is no longer needed for user
  auth, and PHOTO_VIEW_SECRET now signs gallery view tokens.

BETTER_AUTH_SECRET and BETTER_AUTH_URL are required in production; the
deprecated JWT_SECRET stays only as the photo-api view-token fallback.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/.env.example

@@ -50,13 +50,33 @@ DATABASE_URL=./data/spanglish.db
 # Use path-style addressing (true for Garage/MinIO). Defaults to true.
 # S3_FORCE_PATH_STYLE=true
 
-# JWT Secret (change in production!)
+# Better Auth session secret. REQUIRED in production, 32+ characters.
+# Generate one with: openssl rand -base64 48
+# Rotating it signs everyone out (cookie signatures invalidate).
+BETTER_AUTH_SECRET=
+
+# Public site origin Better Auth builds its URLs against (falls back to
+# FRONTEND_URL when unset). E.g. https://spanglishcommunity.com
+BETTER_AUTH_URL=
+
+# Extra reverse-proxy IP prefixes allowed to set X-Real-IP / X-Forwarded-For
+# (comma-separated, e.g. "172.20."). Loopback and RFC1918 ranges are always
+# trusted; anything else is treated as a client and rate-limited by its
+# actual socket address.
+# TRUSTED_PROXIES=
+
+# DEPRECATED: no longer used for API auth (Better Auth replaced the JWTs).
+# Still read by photo-api as the fallback secret for gallery view tokens
+# until PHOTO_VIEW_SECRET is set there; safe to remove after that.
 JWT_SECRET=your-super-secret-key-change-in-production
 
 # Google OAuth (optional - for Google Sign-In)
 # Get your Client ID from: https://console.cloud.google.com/apis/credentials
 # Note: The same Client ID should be used in frontend/.env
 GOOGLE_CLIENT_ID=
+# Only needed for the redirect OAuth flow; the Google Identity Services
+# button (ID token sign-in) works with the Client ID alone.
+GOOGLE_CLIENT_SECRET=
 
 # Server Configuration
 PORT=3001