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/alembic/versions/003_test_versioning.py

@@ -0,0 +1,29 @@
+"""test versioning
+
+Revision ID: 003
+Revises: 002
+Create Date: 2026-03-21
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "003"
+down_revision: Union[str, None] = "002"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "tests",
+        sa.Column("parent_id", sa.Integer(), sa.ForeignKey("tests.id"), nullable=True),
+    )
+    op.create_index("ix_tests_parent_id", "tests", ["parent_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_tests_parent_id", table_name="tests")
+    op.drop_column("tests", "parent_id")