Feature Request: Include bookmark count in `/api/tags/` endpoint

Problem Statement / Use Case

When building third-party integrations, displaying the number of bookmarks associated with each tag is an important UX requirement (e.g., displaying dev (42) or sorting tags by frequency).

Currently, GET /api/tags/ only returns:

{
  "id": 1,
  "name": "example",
  "date_added": "2020-09-26T09:46:23.006313Z"
}

To determine how many bookmarks a tag has, integrations currently have to either:

1. Make a separate GET /api/bookmarks/?q=%23<tag>&limit=1 request for each tag (causing excessive HTTP requests on low-resource self-hosted instances like Raspberry Pi).
2. Or paginate through all bookmarks in the entire instance and compute frequencies client-side.

Proposed Solution

Expose the bookmark count directly on the Tag entity in /api/tags/, for example:

{
  "id": 1,
  "name": "example",
  "date_added": "2020-09-26T09:46:23.006313Z",
  "bookmark_count": 42
}

Performance Considerations

If there are performance concerns about computing counts for users with large datasets by default, this could be made opt-in via a query parameter, for example:

• GET /api/tags/?with_counts=true

Would you be open to considering this enhancement? I'd love to see this feature added if it aligns with the project's roadmap!