feat(library): sort and badge by personal difficulty rating

[mhglover]

Summary

• Adds sort=difficulty / sort=difficulty-desc to the library API — a correlated subquery over song_user_meta.user_difficulty, following the same pattern as the existing mastery sort (unrated songs pushed to the bottom in both directions).
• Surfaces the personal difficulty rating as a ◆N badge on library cards, in both the v2 grid/tree views and the v3 grid.

The rating itself (song_user_meta.user_difficulty) already existed and was editable in the per-song details drawer — this just makes it sortable and visible at a glance in the main library, so it's no longer something you can only see by opening each song individually.

Built as a companion to a new community plugin (difficulty_tagger) that auto-seeds this rating from chart analysis, but this PR has no dependency on it — it works the same whether the rating was set manually or by any plugin.

Test plan

• Full pytest suite passes (2318 passed, 4 skipped)
• Verified live against a real ~700-song library: sort=difficulty-desc correctly surfaces known-hard songs first, sort=difficulty surfaces the built-in beginner tutorial pak first
• Verified the badge renders correctly on both v2 (grid + tree) and v3 grid cards

Summary by CodeRabbit

• New Features
  • Added library sorting by your personal difficulty (“Difficulty (easiest first)” and “Difficulty (hardest first)”).
  • Unrated songs (no difficulty set) are now placed at the bottom in both difficulty sort directions.
  • Songs with a recorded difficulty now show a blue “your difficulty” badge (with tooltip) in both grid and tree/list views, including the tree view.
  • Difficulty sort options are available in the library dropdown and persist across the app.
• Tests
  • Added coverage for difficulty sorting and ensuring the correct difficulty badge/rendering in the tree view.

[coderabbitai]

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📒 Files selected for processing (2)

• server.py
• static/tailwind.min.css

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📝 Walkthrough

Walkthrough

Adds personal difficulty sorting to the library, tree-view song enrichment, and frontend sort options plus difficulty badges in grid and list views.

Changes

Difficulty Sorting Feature

Layer / File(s)	Summary
Backend sort and metadata batching server.py, tests/test_library_filters.py	Adds difficulty sort modes to the library query, batches user difficulty lookups for large filename sets, and verifies unrated songs stay last for both ascending and descending difficulty sorts.
Artist song difficulty enrichment server.py, tests/test_library_filters.py	Attaches user_difficulty to songs returned by query_artists() and tests that the artists endpoint returns the expected rated and unrated values.
Frontend sort options and badges static/app.js, static/index.html, static/v3/songs.js, CHANGELOG.md	Adds difficulty sort options to the UI, accepts the new sort keys, renders difficulty badges when present, and records the feature in the changelog.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main library change: sorting and displaying personal difficulty ratings.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@server.py`:
- Around line 4396-4400: The user_meta_map call in query_artists is batching too
many song IDs into a single IN query and can exceed SQLite’s older variable
limit for prolific artists. Update query_artists to split the rows into the same
400-row batches used by overrides_map, and call user_meta_map per batch before
merging the results so the difficulty badge logic still works without hitting
placeholder limits.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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📥 Commits

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📒 Files selected for processing (4)

• CHANGELOG.md
• server.py
• static/app.js
• tests/test_library_filters.py

✅ Files skipped from review due to trivial changes (1)

• CHANGELOG.md

🚧 Files skipped from review as they are similar to previous changes (1)

• static/app.js