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/requirements.txt

@@ -39,6 +39,11 @@ pytrends==4.9.2
 # Redis client (for health check)
 redis==5.2.1
 
+# Auth
+python-jose[cryptography]==3.3.0
+passlib[bcrypt]==1.7.4
+bcrypt==4.0.1
+
 # Utilities
 python-dateutil==2.9.0
 tiktoken==0.8.0