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>

photo-api/.env.example

@@ -16,8 +16,14 @@ DATABASE_URL=postgresql://spanglish:password@localhost:5432/spanglish_db
 # DB_TYPE=sqlite
 # DATABASE_URL=../backend/data/spanglish.db
 
-# MUST be identical to JWT_SECRET in backend/.env — photo-api validates the
-# same HS256 tokens the backend issues.
+# Secret for gallery image view tokens (HMAC). Any strong random value; does
+# not need to match any backend secret. Rotating it invalidates outstanding
+# image URLs for at most ~1 hour (they are re-minted per page load).
+PHOTO_VIEW_SECRET=8b69468724b730dddecba3d04717cf44e6dc5b808773e7e60156d38875f0f6cd
+
+# DEPRECATED: fallback for PHOTO_VIEW_SECRET during the Better Auth migration
+# (user auth now validates Better Auth sessions from the shared database, not
+# JWTs). Set PHOTO_VIEW_SECRET and remove this.
 JWT_SECRET=
 
 # Public site origin, used to build share links and allow dev CORS