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>

tests/test_library_filters.py

@@ -293,6 +293,45 @@ def test_year_sort_asc_oldest_first(client, seeded):
     assert files == ["b.archive", "a.archive", "f.archive", "d.sloppak", "c.sloppak", "e.sloppak"]
 
 
+def test_difficulty_sort_pushes_unrated_to_bottom(client, server_mod):
+    """Personal difficulty (song_user_meta.user_difficulty) sorts like
+    mastery: an unrated (NULL) row must fall to the bottom in BOTH
+    directions rather than colliding with a real 1..5 rating at either
+    end."""
+    _put(server_mod, filename="easy.archive", title="Easy", artist="A",
+         arrangements=[{"index": 0, "name": "Lead", "notes": 1}])
+    _put(server_mod, filename="hard.archive", title="Hard", artist="B",
+         arrangements=[{"index": 0, "name": "Lead", "notes": 1}])
+    _put(server_mod, filename="unrated.archive", title="Unrated", artist="C",
+         arrangements=[{"index": 0, "name": "Lead", "notes": 1}])
+    server_mod.meta_db.set_song_user_meta("easy.archive", user_difficulty=1)
+    server_mod.meta_db.set_song_user_meta("hard.archive", user_difficulty=5)
+
+    asc = [s["filename"] for s in _get(client, sort="difficulty")["songs"]]
+    assert asc == ["easy.archive", "hard.archive", "unrated.archive"]
+
+    desc = [s["filename"] for s in _get(client, sort="difficulty-desc")["songs"]]
+    assert desc == ["hard.archive", "easy.archive", "unrated.archive"]
+
+
+def test_tree_view_songs_carry_user_difficulty(client, server_mod):
+    """`/api/library/artists` (the classic tree view's `query_artists`) must
+    batch-attach `user_difficulty` the same way `query_page` does for the
+    grid — otherwise the tree view's difficulty badge silently never
+    renders (song.user_difficulty stays undefined for every row)."""
+    _put(server_mod, filename="rated.archive", title="Rated", artist="A",
+         arrangements=[{"index": 0, "name": "Lead", "notes": 1}])
+    _put(server_mod, filename="unrated.archive", title="Unrated", artist="A",
+         arrangements=[{"index": 0, "name": "Lead", "notes": 1}])
+    server_mod.meta_db.set_song_user_meta("rated.archive", user_difficulty=4)
+
+    data = client.get("/api/library/artists").json()
+    songs = data["artists"][0]["albums"][0]["songs"]
+    by_filename = {s["filename"]: s for s in songs}
+    assert by_filename["rated.archive"]["user_difficulty"] == 4
+    assert by_filename["unrated.archive"]["user_difficulty"] is None
+
+
 def test_tuning_sort_down_tuned_before_up_tuned_at_same_distance(client, server_mod):
     """Within an ABS(tuning_sort_key) tier, the down-tuned variant
     must come before the up-tuned one so the order matches the chart's