feat(career): tuning preference filter + interstitial for gigs

Users can now pick a tuning preference before booking a gig:
- Any (default), Standard only, Drop only, or a specific tuning
- Backend filters the song pool (stubs + filler) by that preference
- Empty-filter case returns a 404 with a descriptive message; frontend
  reverts the pref to 'any' and shows a notification
- Interstitial pause before first song and on tuning changes (all prefs
  except 'specific') via window.feedBack.holdAutoplay(); opens the tuner
  panel in auto mode while the user retunes
- 'Specific' gigs skip interstitials (every song already shares one tuning)
- Graceful degradation: no holdAutoplay → interstitial silently skipped

New backend:
- _tuning_ok_fn helper for standard/drop/specific classification
- _fill_genre_songs accepts optional tuning_ok filter
- propose_gig batch-fetches tuning_name for played stubs, applies filter
- GET /gigs/tunings endpoint for the specific-tuning picker

Tests:
- tests/test_career_gig_tuning.py — 17 Python tests (classification, filter)
- tests/js/career_gig_tuning.test.js — 9 JS tests (interstitial logic)
- tests/plugins/career/conftest.py — songs table schema gets tuning_name

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H2bM5jSbMskpdxm2CmuQVj

plugins/career/assets/career.css

@@ -789,3 +789,15 @@
 }
 .pp-giglog-row { font-size: 0.7rem; color: #6d5d40; padding: 0.1rem 0; }
 .pp-giglog-row b { color: #9a5b16; letter-spacing: 0.06em; }
+
+/* Tuning preference pills on gig poster */
+.pp-tuning-row { display: flex; align-items: center; gap: 0.4rem; flex-wrap: wrap; margin: 0.5rem 0 0.3rem; }
+.pp-tuning-pill { font-size: 0.65rem; padding: 0.2rem 0.6rem; border-radius: 999px; opacity: 0.7; }
+.pp-tuning-pill-on { opacity: 1; border-color: #d9a253; color: #d9a253; }
+.pp-tuning-select { font-size: 0.7rem; background: #1a1510; color: #c8b48a; border: 1px solid rgba(138,122,94,0.5); border-radius: 4px; padding: 0.15rem 0.4rem; }
+.pp-poster-tuning-chip { font-size: 0.6rem; color: #8a7a5e; margin-left: 0.35rem; }
+
+/* Gig interstitial — tune up banner */
+.pp-gig-strip.pp-interstitial { pointer-events: auto; display: flex; align-items: center; gap: 0.75rem; border-radius: 8px; border-color: rgba(64,128,224,0.5); }
+.pp-gig-tune-label { color: #4080e0; letter-spacing: 0.08em; font-size: 0.78rem; }
+.pp-gig-start-btn { font-size: 0.7rem; padding: 0.25rem 0.7rem; }