fix(backend): resolve migration path relative to __dirname

migrations glob "src/migrations/**/*.ts" was CWD-relative — broken when CLI runs
outside the src/ directory. Using __dirname makes it absolute and portable.

backend/src/config/data-source.ts

@@ -21,6 +21,6 @@ export const AppDataSource = new DataSource({
     require("../entities/PlaylistVideo").PlaylistVideo,
     require("../entities/PlaylistShare").PlaylistShare,
   ],
-  migrations: ["src/migrations/**/*.ts"],
+  migrations: [__dirname + "/../migrations/**/*.ts"],
   subscribers: [],
 });