feat: add automatic webhook add flow for backends.

[arabcoders]

This pull request introduces a new "Prune" feature for managing and displaying prune handlers (pruners) in the application, as well as a webhook management UI for backends. It also updates dependencies and documentation to reflect these new capabilities. The main changes are grouped below:

Prune Feature (Pruners) Implementation:

• Added a new /prune page (frontend/app/pages/prune.vue) that lists all configured prune handlers, displays their schedule and status, and provides "Dry Run" and "Execute" actions for each pruner. The page includes user tips and reload functionality.
• Updated navigation to include the "Prune" section, making it accessible from the main navigation menu (frontend/app/utils/topLevelNavigation.ts). [1] [2]
• Documented the new API endpoint GET /v1/api/prune in API.md, describing the response structure and usage notes. [1] [2]

Webhook Management for Backends:

• Added a modal dialog and UI controls for adding or updating the webhook URL on remote backends in the backend list page (frontend/app/pages/backends/index.vue). This includes error handling, copy URL functionality, and integration with the new API endpoint. [1] [2] [3] [4] [5]
• Documented the new API endpoint POST /v1/api/backend/{name}/webhook in API.md, including request/response examples, error cases, and backend-specific requirements. [1] [2]