fix claim_fusion MV duplicate-key failure on concurrent refresh

The MV SELECTed COALESCE(user_id, zero-uuid) but GROUPed BY raw user_id,
so a global enrichment claim (user_id NULL) and a default-user behavior
claim (user_id = zero-uuid) for the same edge produced two rows that
collide on idx_claim_fusion_pk, breaking REFRESH ... CONCURRENTLY.
Exposed by the #219 listener_behavior same_scene_as/alias_of writes.

New migration rebuilds the MV grouping by the COALESCE'd user_id so the
two fuse into one row.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/src/services/db.service.ts

@@ -443,6 +443,67 @@ const MIGRATIONS: { id: string; sql: string }[] = [
         ON artists (mbid) WHERE mbid IS NOT NULL;
     `,
   },
+  {
+    id: '20260717_claim_fusion_group_by_coalesce',
+    sql: `
+      -- Fix duplicate-key failures in REFRESH MATERIALIZED VIEW CONCURRENTLY.
+      -- The MV SELECTs COALESCE(user_id, zero-uuid) but GROUPed BY the raw
+      -- user_id, so a global enrichment claim (user_id NULL) and a default-user
+      -- behavior claim (user_id = zero-uuid, e.g. listener_behavior same_scene_as)
+      -- for the same edge fell into separate groups yet collapsed to the same
+      -- output key → two rows violating idx_claim_fusion_pk. Group by the same
+      -- COALESCE'd expression so they fuse into one row.
+      DROP MATERIALIZED VIEW IF EXISTS claim_fusion CASCADE;
+
+      CREATE MATERIALIZED VIEW claim_fusion AS
+      SELECT
+        c.subject_type,
+        c.subject_id,
+        c.predicate,
+        c.object_type,
+        c.object_id,
+        COALESCE(c.user_id, '00000000-0000-0000-0000-000000000000'::uuid) AS user_id,
+        SUM(
+          st.trust * c.confidence *
+          GREATEST(0.1, 1.0 - EXTRACT(DAY FROM NOW() - c.last_reinforced_at) / 180.0)
+        ) AS fused_value,
+        COUNT(*) AS claim_count,
+        MAX(c.last_reinforced_at) AS last_reinforced_at
+      FROM claims c
+      JOIN source_trust st ON st.key = c.source
+      GROUP BY c.subject_type, c.subject_id, c.predicate, c.object_type, c.object_id,
+               COALESCE(c.user_id, '00000000-0000-0000-0000-000000000000'::uuid);
+
+      CREATE UNIQUE INDEX idx_claim_fusion_pk
+        ON claim_fusion (subject_type, subject_id, predicate, object_type, object_id, user_id);
+
+      CREATE OR REPLACE VIEW track_artists_v2 AS
+      SELECT t.id AS track_id,
+             a.id AS artist_id,
+             a.name AS artist_name,
+             CASE cf.predicate WHEN 'credited_main_on' THEN 'main' ELSE 'featured' END AS role,
+             cf.fused_value AS confidence
+      FROM tracks t
+      JOIN claim_fusion cf
+        ON cf.subject_type = 'track' AND cf.subject_id = t.id
+        AND cf.predicate IN ('credited_main_on', 'featured_on')
+        AND cf.object_type = 'artist'
+      JOIN artists a ON a.id = cf.object_id;
+
+      CREATE OR REPLACE VIEW album_artists_v2 AS
+      SELECT al.id AS album_id,
+             a.id AS artist_id,
+             a.name AS artist_name,
+             CASE cf.predicate WHEN 'credited_main_on_album' THEN 'main' ELSE 'featured' END AS role,
+             cf.fused_value AS confidence
+      FROM albums al
+      JOIN claim_fusion cf
+        ON cf.subject_type = 'album' AND cf.subject_id = al.id
+        AND cf.predicate IN ('credited_main_on_album', 'featured_on_album')
+        AND cf.object_type = 'artist'
+      JOIN artists a ON a.id = cf.object_id;
+    `,
+  },
 ];
 
 export class DbService {