ML-169.7: add scrobble stream reset fix task

backlog/tasks/ml-169.7 - Fix-Stream-reset-on-every-scrobble-PubSub-update.md

@@ -0,0 +1,60 @@
+---
+id: ML-169.7
+title: "Fix: Stream reset on every scrobble PubSub update"
+status: To Do
+assignee: []
+created_date: "2026-05-19 11:48"
+labels:
+  - perf
+  - fix
+dependencies: []
+references:
+  - >-
+    backlog/documents/doc-27 -
+    Phase-4-Performance-Audit-Low-Hanging-Fruit-Findings.md
+modified_files:
+  - lib/music_library_web/live/stats_live/index.ex
+  - lib/music_library_web/live/scrobbled_tracks_live/index.ex
+parent_task_id: ML-169
+priority: high
+ordinal: 28000
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+
+Fix performance issue where every scrobble batch (cron: every 5 minutes) triggers a PubSub message that causes StatsLive.Index and ScrobbledTracksLive.Index to reload their entire stream with `reset: true`.
+
+## Problem
+
+Every scrobble batch triggers a `"listening_stats:update"` PubSub message which causes:
+
+- **StatsLive.Index**: Re-executes `recent_activity` (expensive 4-correlated-subquery query) + `scrobble_count`, then resets the `:recent_tracks` and `:recent_albums` streams
+- **ScrobbledTracksLive.Index**: Re-executes the full paginated list query and resets the `:tracks` stream
+
+This fires every 5 minutes on ALL connected clients, even idle ones.
+
+## Files
+
+- `lib/music_library_web/live/stats_live/index.ex:576-583` — `assign_scrobble_activity` with `reset: true`
+- `lib/music_library_web/live/scrobbled_tracks_live/index.ex:279` — `handle_info(%{track_count: _count})`
+- `lib/music_library_web/live/scrobbled_tracks_live/index.ex:334` — `load_and_assign_tracks` with `reset: true`
+
+## Fix
+
+- **StatsLive**: Use `stream_insert` for new tracks instead of `reset: true`. Keep a cursor (e.g., max `scrobbled_at_uts`) to only insert genuinely new tracks.
+- **ScrobbledTracksLive**: Use `stream_insert` for new tracks only if they match the current search/filter. Otherwise, show a "new scrobbles" badge that refreshes on user click.
+- Consider debouncing or batching the updates to avoid per-track insert thrash.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+
+<!-- AC:BEGIN -->
+
+- [ ] #1 StatsLive no longer performs full stream reset on scrobble updates
+- [ ] #2 Only new tracks are inserted into streams
+- [ ] #3 ScrobbledTracksLive no longer performs full stream reset on scrobble updates
+- [ ] #4 Existing stream ordering is preserved after insert
+- [ ] #5 No duplicate tracks appear in streams
+<!-- AC:END -->