Refactor Alembic setup and add migration helper script.

Updated Alembic configuration and folder structure to reference the `app` module. Introduced a new `migrate.py` script to manage migrations efficiently with commands for generating, applying, and inspecting migrations. Adjusted `env.py` to ensure proper model imports and use environment-driven database URLs.

backend/alembic.ini

@@ -1,5 +1,5 @@
 [alembic]
-script_location = alembic
+script_location = app/alembic
 sqlalchemy.url = postgresql://postgres:postgres@db:5432/eventspace
 
 [loggers]