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/main.py

@@ -1,7 +1,7 @@
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from app.api import bills, members, follows, dashboard, search, settings, admin, health
+from app.api import bills, members, follows, dashboard, search, settings, admin, health, auth
 from app.config import settings as config
 
 app = FastAPI(
@@ -18,6 +18,7 @@ app.add_middleware(
     allow_headers=["*"],
 )
 
+app.include_router(auth.router, prefix="/api/auth", tags=["auth"])
 app.include_router(bills.router, prefix="/api/bills", tags=["bills"])
 app.include_router(members.router, prefix="/api/members", tags=["members"])
 app.include_router(follows.router, prefix="/api/follows", tags=["follows"])