feat: add Alembic

This adds the `ratings_index` to code, which so far wasn't reflected in
SQLAlchemy's metadata.

alembic.ini

@@ -0,0 +1,39 @@
+[alembic]
+script_location = alembic
+file_template = %%(epoch)s-%%(rev)s_%%(slug)s
+timezone = UTC
+
+# Logging configuration
+[loggers]
+keys = root,sqlalchemy,alembic
+
+[handlers]
+keys = console
+
+[formatters]
+keys = generic
+
+[logger_root]
+level = WARN
+handlers = console
+qualname =
+
+[logger_sqlalchemy]
+level = WARN
+handlers =
+qualname = sqlalchemy.engine
+
+[logger_alembic]
+level = INFO
+handlers =
+qualname = alembic
+
+[handler_console]
+class = StreamHandler
+args = (sys.stderr,)
+level = NOTSET
+formatter = generic
+
+[formatter_generic]
+format = %(levelname)-5.5s [%(name)s] %(message)s
+datefmt = %H:%M:%S