feat: optimized board queries

[psychedelicious]

Summary

In #6603 I made some changes to reduce SQL queries and network requests when listing boards. That PR got crusty, this PR updates it to work on main.

Two main changes.

feat(app): add method & route to get uncategorized image counts

Previously we were abusing the list boards query (twice!) to get the counts for uncategorized images/assets. There is now a dedicated endpoint for this.

feat(app): optimize boards queries

Use SQL instead of python to retrieve image count, asset count and board cover image.

This reduces the number of SQL queries needed to list all boards. Previously, we did 1 + 2 * board_count queries::

• 1 query to get the list of board records
• 1 query per board to get its total count
• 1 query per board to get its cover image

Then, on the frontend, we made two additional network requests to get each board's counts:

• 1 request (== 1 SQL query) for image count
• 1 request (== 1 SQL query) for asset count

All of this information is now retrieved in a single SQL query, and provided via single network request.

As part of this change, BoardRecord now includes image_count, asset_count and cover_image_name. This makes BoardDTO redundant, but removing it is a deeper change...

Related Issues / Discussions

• Supersedes #6603 (this is that PR, but updated for main)

QA Instructions

Needs re-review from @maryhipp.

Board counts should be correct and update as images are moved between boards and generated/saved to gallery.

Merge Plan

This should only be merged after v5 is released.

Checklist

• [x] The PR has a short but descriptive title, suitable for a changelog
• [ ] Tests added / updated (if applicable)
• [ ] Documentation added / updated (if applicable)