refactor: use backend API for user ID uniqueness validation

- Add GET /api/v1/users/check-id/{user_id} endpoint for real-time validation
- Replace frontend local validation with debounced API calls (500ms)
- Support exclude_user_id parameter for edit scenarios
- Add UserIdCheckResponse schema
- Maintain format validation (alphanumeric + underscore) on frontend
- Clean up timeout on component unmount

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: multica-agent <github@multica.ai>

api/app/api/v1/users.py

@@ -11,6 +11,7 @@ from ...schemas.user import (
     UserPublic,
     UserRoleUpdateRequest,
     UserUpdateRequest,
+    UserIdCheckResponse,
 )
 from ...services.user_service import (
     UserCreateError,
@@ -29,6 +30,24 @@ from ...services.user_service import (
 router = APIRouter(prefix="/users", tags=["users"])
 
 
+@router.get("/check-id/{user_id}", response_model=UserIdCheckResponse)
+def check_user_id_availability(
+    user_id: str,
+    exclude_user_id: str | None = Query(default=None),
+    _: CurrentUser = Depends(require_permission("user.manage")),
+    db: Session = Depends(get_db),
+) -> UserIdCheckResponse:
+    """Check if a user ID is available. Use exclude_user_id when editing an existing user."""
+    existing_user = get_user_by_id(db, user_id)
+
+    if existing_user:
+        if exclude_user_id and existing_user.id.lower() == exclude_user_id.lower():
+            return UserIdCheckResponse(available=True, message="Current user ID")
+        return UserIdCheckResponse(available=False, message="User ID already exists")
+
+    return UserIdCheckResponse(available=True, message="User ID is available")
+
+
 @router.post("", response_model=UserPublic)
 def create_user_account(
     payload: UserCreateRequest,

api/app/schemas/user.py

@@ -20,6 +20,11 @@ class UserListResponse(BaseModel):
     total: int
 
 
+class UserIdCheckResponse(BaseModel):
+    available: bool
+    message: str
+
+
 class UserUpdateRequest(BaseModel):
     email: str | None = None
     username: str | None = Field(default=None, min_length=3, max_length=64)