feat: Sprint 3 — test editing with versioning

Backend:
- migration 003: add parent_id to tests table
- PUT /api/tests/{id}: edit in place if no attempts, create new version otherwise
- GET /api/tests: show only latest versions (no successor)

Frontend:
- TestForm: extracted reusable form component
- TestCreate: refactored to use TestForm
- TestEdit: full edit mode with pre-populated form, version redirect on new version

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

backend/app/schemas/test.py

@@ -65,12 +65,18 @@ class TestOut(BaseModel):
     allow_navigation_back: bool
     is_active: bool
     version: int
+    parent_id: Optional[int]
     created_at: datetime
     questions: list[QuestionOut] = []
 
     model_config = {"from_attributes": True}
 
 
+class TestUpdateResponse(BaseModel):
+    test: TestOut
+    is_new_version: bool
+
+
 class TestListItem(BaseModel):
     id: int
     title: str