feat(taste): household co-play similarity — #1533

Milestone #160 Opt 5. A collaborative candidate arm: tracks by artists
co-played across the instance with the seed's artist.

Minstrel is a single shared-library, multi-user server (no per-user
library ACL — verified: no owner/share/group model), so the "household"
is the whole instance's user set; the rule #47 scoping is satisfied by
the shared-library boundary. Single-user servers produce no edges.

- No migration: source='user_cooccurrence' was pre-whitelisted in the
  0009 similarity CHECK from day one.
- internal/db/queries/coplay.sql: Delete + Insert artist co-play edges.
  Score = Jaccard of the two artists' distinct-player sets (controls for
  globally-popular artists); >= 2 co-players AND Jaccard >= floor kept
  (the floor also self-limits hub artists). Completed plays, 365d window.
- internal/coplay: periodic worker (6h) that atomic-replaces the
  user_cooccurrence edge set from play_events — pure local SQL, no
  external calls. Wired in main.go alongside the similarity worker.
- LoadRadioCandidatesV2: new coplay_artists arm (source='user_cooccurrence',
  seed-artist based, 0.5 damp like similar_artists) + $11 limit;
  CandidateSourceLimits.UserCoplay (default 20, For-You 40).
- Integration tests: perfect-overlap Jaccard=1.0 edge + single-user
  empty-set gate.

Device axis and AcousticBrainz (Opt 4) are separately tracked; this
closes the milestone-#160 sequential options.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/db/queries/recommendation.sql

@@ -42,7 +42,9 @@ WHERE t.id <> $2
 --   $10 taste_overlap K (#796 phase 2b — tracks by the user's top
 --   positively-weighted taste-profile artists, so taste-relevant tracks
 --   enter the pool even when the similarity/random arms miss them; scored
---   in Go via TasteMatch, so sim_score here is 0 pool-inclusion).
+--   in Go via TasteMatch, so sim_score here is 0 pool-inclusion),
+--   $11 coplay_artists K (#1533 — tracks by artists co-played across the
+--   instance with the seed's artist; source='user_cooccurrence').
 -- Returns same shape as LoadRadioCandidates plus similarity_score column.
 
 WITH
@@ -126,6 +128,22 @@ taste_overlap AS (
     ORDER BY tpa.weight DESC, t.id
     LIMIT $10
 ),
+coplay_artists AS (
+    -- Household co-play (#1533): tracks by artists co-played across the
+    -- instance with the seed's artist (source='user_cooccurrence', built by
+    -- the coplay worker). Mirrors similar_artists but from local co-occurrence
+    -- instead of ListenBrainz; empty on single-user servers. Same 0.5 damp as
+    -- similar_artists since it's artist-level.
+    SELECT t.id AS track_id, asim.score * 0.5 AS sim_score
+    FROM artist_similarity asim
+    JOIN tracks t ON t.artist_id = asim.artist_b_id
+    JOIN seed_artist sa ON asim.artist_a_id = sa.artist_id
+    WHERE asim.source = 'user_cooccurrence'
+      AND t.id NOT IN (SELECT id FROM excluded_ids)
+      AND t.id <> $2
+    ORDER BY asim.score DESC, random()
+    LIMIT $11
+),
 random_fill AS (
     SELECT t.id AS track_id, 0.0::float8 AS sim_score
     FROM tracks t
@@ -137,6 +155,7 @@ random_fill AS (
           UNION SELECT track_id FROM tag_overlap
           UNION SELECT track_id FROM likes_overlap
           UNION SELECT track_id FROM taste_overlap
+          UNION SELECT track_id FROM coplay_artists
       )
     ORDER BY random()
     LIMIT $9
@@ -155,6 +174,7 @@ FROM (
     UNION ALL SELECT track_id, sim_score FROM tag_overlap
     UNION ALL SELECT track_id, sim_score FROM likes_overlap
     UNION ALL SELECT track_id, sim_score FROM taste_overlap
+    UNION ALL SELECT track_id, sim_score FROM coplay_artists
     UNION ALL SELECT track_id, sim_score FROM random_fill
 ) u
 JOIN tracks t ON t.id = u.track_id