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/models/test.py

@@ -18,6 +18,9 @@ class Test(Base):
     allow_navigation_back: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
     is_active: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
     version: Mapped[int] = mapped_column(Integer, default=1, nullable=False)
+    parent_id: Mapped[int | None] = mapped_column(
+        Integer, ForeignKey("tests.id"), nullable=True
+    )
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now()
     )