feat(library): sort and badge by personal difficulty rating (#810)

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

Adds sort=difficulty/difficulty-desc to the library API (correlated
subquery over song_user_meta.user_difficulty, unrated songs pushed to
the bottom either direction, same pattern as the existing mastery
sort) and surfaces the rating as a badge on library cards in both the
v2 grid/tree views and the v3 grid. The rating itself already existed
(song_user_meta) — this just makes it sortable and visible, so it's
no longer only readable in the per-song edit drawer.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* fix(library): escape difficulty badge, wire tree view, add changelog+tests

- Wrap song.user_difficulty in esc() at both badge call sites
  (static/app.js ~2082 and ~2283) for XSS-consistency with the
  sibling tuning badge, which already uses esc().
- server.py: query_artists (the classic tree view's data source, used
  by /api/library/artists) never batch-attached user_difficulty the
  way query_page does for the grid, so the tree-view difficulty badge
  added in 75673c3 was unreachable dead code (song.user_difficulty was
  always undefined there). Now attaches it via the existing
  user_meta_map() helper, same pattern as query_page.
- Add an [Unreleased] CHANGELOG.md entry for the difficulty sort +
  badge feature, matching the repo's existing entry format.
- Add tests/test_library_filters.py::test_difficulty_sort_pushes_unrated_to_bottom
  asserting unrated songs sort to the bottom in both sort=difficulty
  and sort=difficulty-desc directions, and
  ::test_tree_view_songs_carry_user_difficulty covering the
  query_artists fix above.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(library): chunk user_meta_map + rebuild stale tailwind css

Address review-bot findings on the difficulty sort/badge:
- user_meta_map now chunks filenames into 400-row batches (like
  overrides_map) before the IN (...) query. query_artists (tree view)
  passes every song across up to 50 artists, which could push the
  placeholder count past SQLite's older variable limit; query_page's
  small pages are unaffected. (CodeRabbit: Stability & Availability)
- Rebuild static/tailwind.min.css: the ◆N difficulty badge introduced
  bg-blue-900/30 + text-blue-300, which were never compiled into the
  committed stylesheet, failing the tailwind-fresh CI gate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: byrongamatos <xasiklas@gmail.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Sort and filter the library by your personal difficulty rating — now visible at a glance, not just in the edit drawer.** `song_user_meta.user_difficulty` (the 1–5 planning rating, settable manually or seeded by a plugin like the community `difficulty_tagger`) already existed but was only readable by opening a song's per-song details drawer. The library API gains `sort=difficulty` / `sort=difficulty-desc` — a correlated subquery over `song_user_meta`, following the same unrated-rows-sort-to-the-bottom-in-both-directions pattern as the existing `mastery` sort — and library cards now show the rating as a `◆N` badge (v2 grid/tree views and the v3 grid alike), next to the tuning and lyrics badges. The classic tree view's `query_artists` batch-attaches `user_difficulty` the same way `query_page` already did for the grid, so the badge actually renders there too instead of staying dark. Tests: `tests/test_library_filters.py::test_difficulty_sort_pushes_unrated_to_bottom`, `tests/test_library_filters.py::test_tree_view_songs_carry_user_difficulty`.
 - **`lib/midi_import.py`: `convert_midi_tempo_map` — MIDI imports can finally carry
   their bars.** The keys/drums note converters always computed a tempo-aware
   tick→seconds map internally (to bake note times to absolute seconds) and then threw