feat: expose admin status to frontend via user API

Add is_admin field to UserResponse schema and update AuthContext to
fetch user profile after login, storing and exposing isAdmin boolean.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/app/api/users.py

@@ -16,6 +16,7 @@ class UserResponse(CamelModel):
     id: UUID
     email: str
     display_name: str | None = None
+    is_admin: bool = False
 
 
 @router.post("/me", response_model=UserResponse)