Send push notification

Send a push notification to a device token or all devices of a user. **Features:** - Send to a specific device token OR all devices of a userId - Support for Android, iOS, and Web platforms - Rich notifications with images, custom data, and actions - Priority and TTL settings - Badge and sound customization - Automatically saves notification to database **Target Options:** - **token**: Send to a specific device token (optional) - **userId**: Send to all registered devices of a user (optional) - At least one of `token` or `userId` must be provided **Authentication:** - Requires valid Bearer token - Test tokens available for development: `test-jwt-token`, `agent-jwt-token` **Platform Support:** - **Android**: FCM with custom sounds, click actions, and TTL - **iOS**: APNs with badge, sound, and custom payload - **Web**: Web Push with icons and click actions **Examples:** - Send to specific device: Provide `token` field - Send to all user devices: Provide `userId` field (fetches all tokens from database) - Rich notification: Include image, custom data, and click action - High priority: Set priority to "high" for time-sensitive notifications