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/models/__init__.py

@@ -6,6 +6,7 @@ from app.models.news import NewsArticle
 from app.models.setting import AppSetting
 from app.models.trend import TrendScore
 from app.models.committee import Committee, CommitteeBill
+from app.models.user import User
 
 __all__ = [
     "Bill",
@@ -19,4 +20,5 @@ __all__ = [
     "TrendScore",
     "Committee",
     "CommitteeBill",
+    "User",
 ]