Add user management (CRUD + admin toggle), make Settings available to all users

- New /admin/users page: create/delete users, toggle admin role, guarded against
  self-demotion/self-deletion
- Backend: GET/POST/PATCH/DELETE /api/admin/users, admin-only
- Settings moved from /admin/settings to /settings (all logged-in users); the
  password-change section shows for everyone, integration/API-key sections only
  render (and only fetch) for admins, since those endpoints stay admin-gated
- Navbar: Settings link now visible to all users; added Users nav link for admins

backend/models.py

@@ -34,6 +34,19 @@ class UserPublic(BaseModel):
     created_at: str
 
 
+class AdminUserCreate(BaseModel):
+    email: str
+    password: str
+    name: str
+    is_admin: bool = False
+
+
+class AdminUserUpdate(BaseModel):
+    name: Optional[str] = None
+    is_admin: Optional[bool] = None
+    password: Optional[str] = None
+
+
 # ---------- Profiles ("Who's Watching") ----------
 RATING_ORDER = ["G", "PG", "PG-13", "R", "NR"]