fix migration

backend/app/db/migrate_fitness.py

@@ -29,12 +29,19 @@ def _add_column_if_missing(table: str, column: str, ddl: str) -> None:
 
 
 def _ensure_schema_migrations_table() -> None:
+    from app.db.dialect import is_postgresql
+
+    applied_type = (
+        "TIMESTAMP DEFAULT CURRENT_TIMESTAMP"
+        if is_postgresql(engine)
+        else "DATETIME DEFAULT CURRENT_TIMESTAMP"
+    )
     with engine.begin() as conn:
         conn.execute(
             text(
                 "CREATE TABLE IF NOT EXISTS _schema_migrations ("
                 "name TEXT PRIMARY KEY, "
-                "applied_at DATETIME DEFAULT CURRENT_TIMESTAMP)"
+                f"applied_at {applied_type})"
             )
         )