fix(api): centralize and correct dotenv loading

Refactors environment variable handling to be self-contained within the `EnvironmentConfig` class.

- The `dotenv` loading logic is moved from `AppDependencies` to `EnvironmentConfig`.
- `EnvironmentConfig` now creates a static `DotEnv` instance, loads the `.env` file, and all getters now read from this instance instead of `Platform.environment`.
- This ensures that environment variables are loaded and read correctly from a single, reliable source, resolving the issue where variables were not available to the application.

Author: fulleni

lib/src/config/app_dependencies.dart

@@ -1,6 +1,5 @@
 import 'dart:async';
 
-import 'package:dotenv/dotenv.dart';
 import 'package:ht_api/src/config/database_connection.dart';
 import 'package:ht_api/src/rbac/permission_service.dart';
 import 'package:ht_api/src/services/auth_service.dart';
@@ -83,10 +82,6 @@ class AppDependencies {
   }
 
   Future<void> _init() async {
-    // 0. Load environment variables from .env file.
-    DotEnv(includePlatformEnvironment: true).load();
-    _log.info('Environment variables loaded from .env file.');
-
     // 1. Establish Database Connection.
     await DatabaseConnectionManager.instance.init();
     final connection = await DatabaseConnectionManager.instance.connection;