Smart Playlist Seeding & Per-User Play Tracking

Summary

This PR implements the full feature plan from PLAN.md — per-user implicit rating system, smarter playlist generation with preference-based reseeding, onboarding cadence, and a Playwright e2e test suite.

What Changed

Phase 1 — Per-User Play/Skip Tracking (schema)

• UserTrackStats model: per-user playCount and skipCount per track
• Rating.implicit field: distinguishes system-generated ratings from explicit user choices
• Removed global Track.playCount (was shared across all users, now per-user in UserTrackStats)
• Prisma migration included

Phase 2 — Implicit Rating Service

• implicitRatingService.js: evaluates net = playCount - skipCount per user/track
  • net ≥ 5 → implicit Like; net ≤ -5 → implicit Dislike; neutral zone → remove implicit rating
• /api/tracks/complete now upserts UserTrackStats and calls evaluateImplicitRating
• /api/tracks/skip new route — records skip event and re-evaluates implicit rating
• /api/ratings deletes any implicit rating before upserting an explicit one (explicit always wins)

Phase 3 — Player Skip Detection

• 80% completion threshold fires /complete
• Skip is detected if elapsed ≥ 5s AND currentTime/duration < 20% — fires /skip
• All navigation paths (prev/next buttons, keyboard, media keys) run skip detection

Phase 4 — Onboarding Cadence

• Two-step onboarding modal: genre picker → cadence picker (daily / weekly / monthly)
• /api/onboarding/seed accepts { genres, cadence } and upserts CadenceSetting

Smart Playlist Generation (new)

• Liked tracks are never dropped: explicit or implicit score > 0 always included
• Rotation by play count: when liked tracks exceed 70% of cycle size, least-played liked songs are prioritised so every favourite gets airtime over successive cycles
• 30% discovery slots: always reserved for neutral/unrated tracks to surface new music
• Liked tracks protected from file purge: cleanupService now skips any track with a positive rating before deleting audio files

Preference-Based Reseeding

• lastfmService: added fetchSimilarArtists
• schedulerService: smarter seedFromLastfm uses UserTrackStats + ratings to build artist/genre preference profile; expands via similar artists; library soft cap of 250 tracks

Bug Fixes

• Profile menu z-index (appeared behind page content)
• DebugPage background (invalid CSS gradient syntax)
• DownloadPanel progress counter (X / Y done)
• Rate limiting removed (family-hosted service, not public-facing)

Playwright E2E Test Suite (20 tests, all passing)

• e2e/auth.spec.js — registration, login, error states
• e2e/onboarding.spec.js — genre selection, cadence step, back nav, seeding
• e2e/app.spec.js — profile menu z-index, debug page, download panel, player bar

Testing

# Run all e2e tests (requires dev servers running)
npm test

# or with auto-start
START_SERVERS=1 npm test

All 20 Playwright tests pass locally.