Add multi-user auth system and admin panel

- User model with email/hashed_password/is_admin/notification_prefs
- JWT auth: POST /api/auth/register, /login, /me
- First registered user auto-promoted to admin
- Migration 0005: users table + user_id FK on follows (clears global follows)
- Follows, dashboard, settings, admin endpoints all require authentication
- Admin endpoints (settings writes, celery triggers) require is_admin
- Frontend: login/register pages, Zustand auth store (localStorage persist)
- AuthGuard component gates all app routes, shows app shell only when authed
- Sidebar shows user email + logout; Admin nav link visible to admins only
- Admin panel (/settings): user list with delete + promote/demote, LLM config,
  data source settings, and manual celery controls

Authored-By: Jack Levy

backend/app/config.py

@@ -9,6 +9,10 @@ class Settings(BaseSettings):
     LOCAL_URL: str = "http://localhost"
     PUBLIC_URL: str = ""
 
+    # Auth / JWT
+    JWT_SECRET_KEY: str = "change-me-in-production"
+    JWT_EXPIRE_MINUTES: int = 60 * 24 * 7  # 7 days
+
     # Database
     DATABASE_URL: str = "postgresql+asyncpg://congress:congress@postgres:5432/pocketveto"
     SYNC_DATABASE_URL: str = "postgresql://congress:congress@postgres:5432/pocketveto"