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/core/dependencies.py

@@ -0,0 +1,41 @@
+from fastapi import Depends, HTTPException, status
+from fastapi.security import OAuth2PasswordBearer
+from jose import JWTError
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.security import decode_token
+from app.database import get_db
+from app.models.user import User
+
+oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/api/auth/login")
+
+
+async def get_current_user(
+    token: str = Depends(oauth2_scheme),
+    db: AsyncSession = Depends(get_db),
+) -> User:
+    credentials_error = HTTPException(
+        status_code=status.HTTP_401_UNAUTHORIZED,
+        detail="Invalid or expired token",
+        headers={"WWW-Authenticate": "Bearer"},
+    )
+    try:
+        user_id = decode_token(token)
+    except JWTError:
+        raise credentials_error
+
+    user = await db.get(User, user_id)
+    if user is None:
+        raise credentials_error
+    return user
+
+
+async def get_current_admin(
+    current_user: User = Depends(get_current_user),
+) -> User:
+    if not current_user.is_admin:
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail="Admin access required",
+        )
+    return current_user

backend/app/core/security.py

@@ -0,0 +1,36 @@
+from datetime import datetime, timedelta, timezone
+
+from jose import JWTError, jwt
+from passlib.context import CryptContext
+
+from app.config import settings
+
+pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
+
+ALGORITHM = "HS256"
+
+
+def hash_password(password: str) -> str:
+    return pwd_context.hash(password)
+
+
+def verify_password(plain: str, hashed: str) -> bool:
+    return pwd_context.verify(plain, hashed)
+
+
+def create_access_token(user_id: int) -> str:
+    expire = datetime.now(timezone.utc) + timedelta(minutes=settings.JWT_EXPIRE_MINUTES)
+    return jwt.encode(
+        {"sub": str(user_id), "exp": expire},
+        settings.JWT_SECRET_KEY,
+        algorithm=ALGORITHM,
+    )
+
+
+def decode_token(token: str) -> int:
+    """Decode JWT and return user_id. Raises JWTError on failure."""
+    payload = jwt.decode(token, settings.JWT_SECRET_KEY, algorithms=[ALGORITHM])
+    user_id = payload.get("sub")
+    if user_id is None:
+        raise JWTError("Missing sub claim")
+    return int(user_id)