Adds the first quality feature on top of the manager: when
`config.exclude_recent_days > 0`, the manager drops any track from
the generator's output whose primary id was served by this kind
for this profile in the last N days.
Lives at the manager layer, not in each generator, so:
- generators stay focused on selection logic
- staleness behavior stays uniform across every kind
- enabling/disabling per playlist is just a config patch
Implementation:
- New `PersonalizedPlaylistManager._apply_quality_filters` runs after
generator returns, before `_persist_snapshot`.
- Reads recent ids via existing `recent_track_ids` accessor.
- Tracks without a primary id pass through unchanged (nothing to
dedupe on -- happens for sourceless tracks during edge cases).
- Returns a new list (never mutates input).
Default `exclude_recent_days = 0` preserves pre-overhaul behavior.
Per-playlist override via `PUT /api/personalized/playlist/<kind>/config`
with `{"exclude_recent_days": N}`. Recommended values:
- Discovery Shuffle: 1-3 days (high churn desired)
- Hidden Gems: 7-14 days (avoid same gems weekly)
- Time Machine / Genre: 30+ days (slow rotation OK, stable view preferred)
4 new boundary tests:
- Zero days = no filter (default behavior preserved)
- Positive days drops tracks served in window
- Filter preserves new tracks alongside dropped ones
- Tracks without primary id pass through unchanged
3369 tests pass total.
Note: listening-history cross-ref + seeded shuffle are deferred to
a future PR. Each requires deeper integration -- listening history
needs a play-events table the discovery pool can query against;
seeded shuffle needs the legacy generators to accept a seed param
without breaking their existing diversity / popularity logic.
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