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/api/auth.py

@@ -0,0 +1,58 @@
+from fastapi import APIRouter, Depends, HTTPException, status
+from sqlalchemy import func, select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.dependencies import get_current_user
+from app.core.security import create_access_token, hash_password, verify_password
+from app.database import get_db
+from app.models.user import User
+from app.schemas.schemas import TokenResponse, UserCreate, UserResponse
+
+router = APIRouter()
+
+
+@router.post("/register", response_model=TokenResponse, status_code=201)
+async def register(body: UserCreate, db: AsyncSession = Depends(get_db)):
+    if len(body.password) < 8:
+        raise HTTPException(status_code=400, detail="Password must be at least 8 characters")
+    if "@" not in body.email:
+        raise HTTPException(status_code=400, detail="Invalid email address")
+
+    # Check for duplicate email
+    existing = await db.execute(select(User).where(User.email == body.email.lower()))
+    if existing.scalar_one_or_none():
+        raise HTTPException(status_code=409, detail="Email already registered")
+
+    # First registered user becomes admin
+    count_result = await db.execute(select(func.count()).select_from(User))
+    is_first_user = count_result.scalar() == 0
+
+    user = User(
+        email=body.email.lower(),
+        hashed_password=hash_password(body.password),
+        is_admin=is_first_user,
+    )
+    db.add(user)
+    await db.commit()
+    await db.refresh(user)
+
+    return TokenResponse(access_token=create_access_token(user.id), user=user)
+
+
+@router.post("/login", response_model=TokenResponse)
+async def login(body: UserCreate, db: AsyncSession = Depends(get_db)):
+    result = await db.execute(select(User).where(User.email == body.email.lower()))
+    user = result.scalar_one_or_none()
+
+    if not user or not verify_password(body.password, user.hashed_password):
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Incorrect email or password",
+        )
+
+    return TokenResponse(access_token=create_access_token(user.id), user=user)
+
+
+@router.get("/me", response_model=UserResponse)
+async def me(current_user: User = Depends(get_current_user)):
+    return current_user