feat: sqlite migration runner with meta version table

tests/test_migrate.py

@@ -0,0 +1,22 @@
+from chat.db.connection import open_db
+from chat.db.migrate import apply_migrations
+
+
+def test_apply_migrations_creates_meta_table(tmp_path):
+    db = tmp_path / "test.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        row = conn.execute(
+            "SELECT value FROM meta WHERE key = 'schema_version'"
+        ).fetchone()
+        assert row is not None
+        assert int(row[0]) >= 1
+
+
+def test_apply_migrations_idempotent(tmp_path):
+    db = tmp_path / "test.db"
+    apply_migrations(db)
+    apply_migrations(db)  # second call must be a no-op
+    with open_db(db) as conn:
+        count = conn.execute("SELECT COUNT(*) FROM meta").fetchone()[0]
+        assert count == 1