Delete push notification token

Delete a registered push notification token. **Features:** - Remove device token from all collections - Clean up topic subscriptions - User ownership verification - Complete token removal **Authentication:** - Requires valid Bearer token - Users can only delete their own tokens - Test tokens available for development: `test-jwt-token`, `agent-jwt-token` **Use Cases:** - User logout and token cleanup - Device uninstallation - Security token revocation - Privacy compliance (GDPR) **Token Cleanup:** - Removes from user's token list - Removes from platform-specific collections - Removes from device-specific collections - Unsubscribes from all topics - Cleans up Redis cache **Example Request:** ``` DELETE /notifications/tokens/token_123 Authorization: Bearer your-jwt-token ```