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/user.py

@@ -0,0 +1,19 @@
+from sqlalchemy import Boolean, Column, DateTime, Integer, String
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+
+from app.database import Base
+
+
+class User(Base):
+    __tablename__ = "users"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    email = Column(String, unique=True, nullable=False, index=True)
+    hashed_password = Column(String, nullable=False)
+    is_admin = Column(Boolean, nullable=False, default=False)
+    notification_prefs = Column(JSONB, nullable=False, default=dict)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    follows = relationship("Follow", back_populates="user", cascade="all, delete-orphan")