feat(api): add dotenv dependency

Adds the `dotenv` package to manage environment variables directly within the application. This makes the server's configuration self-contained and less reliant on the execution environment of the `dart_frog_cli` tool.

Author: fulleni

pubspec.yaml

@@ -9,6 +9,7 @@ environment:
 dependencies:
   dart_frog: ^1.1.0
   dart_jsonwebtoken: ^3.2.0
+  dotenv: ^4.2.0
   ht_data_client:
     git:
       url: https://github.com/headlines-toolkit/ht-data-client.git