Add runtime DB schema migration and version tracking

backend/tests/test_repositories/test_db_init.py

@@ -67,3 +67,34 @@ async def test_init_db_is_idempotent(tmp_path: Path) -> None:
     async with aiosqlite.connect(db_path) as db:
         await init_db(db)
         await init_db(db)  # Second call must be a no-op.
+
+
+@pytest.mark.asyncio
+async def test_init_db_records_schema_version(tmp_path: Path) -> None:
+    """``init_db`` must record the current schema version."""
+    db_path = str(tmp_path / "test.db")
+    async with aiosqlite.connect(db_path) as db:
+        await init_db(db)
+        async with db.execute(
+            "SELECT version FROM schema_migrations ORDER BY version DESC LIMIT 1;"
+        ) as cursor:
+            row = await cursor.fetchone()
+    assert row is not None
+    assert row[0] == 1
+
+
+@pytest.mark.asyncio
+async def test_init_db_migrates_legacy_database_without_schema_version(tmp_path: Path) -> None:
+    """Legacy databases without a schema marker must be migrated on startup."""
+    db_path = str(tmp_path / "test.db")
+    async with aiosqlite.connect(db_path) as db:
+        await init_db(db)
+        await db.execute("DROP TABLE schema_migrations;")
+        await db.commit()
+        await init_db(db)
+        async with db.execute(
+            "SELECT version FROM schema_migrations ORDER BY version DESC LIMIT 1;"
+        ) as cursor:
+            row = await cursor.fetchone()
+    assert row is not None
+    assert row[0] == 1