feat(users): add admin-safe user management endpoints

- require admin auth for user CRUD
- support editable email/full name/password/admin/active fields
- prevent self lockout and self deletion
- return clear error when related records block deletion

app/schemas/schemas.py

@@ -164,6 +164,14 @@ class UserCreate(UserBase):
     is_admin: bool = False
 
 
+class UserUpdate(BaseModel):
+    full_name: Optional[str] = None
+    email: Optional[str] = None
+    password: Optional[str] = None
+    is_admin: Optional[bool] = None
+    is_active: Optional[bool] = None
+
+
 class UserResponse(UserBase):
     id: int
     is_active: bool