bark-rest: add notification long polling endpoint

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Summary by CodeRabbit

• New Features
  • Added long-poll REST endpoint (GET /api/v1/notifications/wait) for wallet notifications, with optional since filtering and responses containing notifications and last_pushed_at for resumable polling.
• API Improvements
  • Added support for 422 Unprocessable Entity responses for invalid/unsupported request inputs.
• Documentation
  • Updated OpenAPI spec and REST client docs/models to include the new endpoint and WaitNotificationResponse.
• Tests
  • Added long-poll integration tests verifying authenticated delivery, last_pushed_at, and unauthenticated 401 behavior.
• Refactor
  • Improved server shutdown coordination to shut down components consistently.