fix: stop the deleted_permanent audit row from destroying itself

db.service inserted the 'deleted_permanent' feedback row and then deleted
the track, but feedback.track_id was ON DELETE CASCADE (verified on the
live DB: confdeltype = 'c'), so the audit row deleted itself. feedback
contains zero deleted_permanent rows.

feedback is an audit log and must outlive its subject: the FK becomes
ON DELETE SET NULL. track_id was already nullable, and nothing in backend/
or workers/ SELECTs from feedback — the only other reference is
mergeTracks()'s UPDATE feedback SET track_id, which re-points to the
survivor — so no caller assumed non-null.

Migration 20260730_feedback_track_id_set_null drops the constraint by
matching confdeltype rather than by name, since the live schema has
drifted. Verified on a scratch PG16: confdeltype flips 'c' -> 'n' and a
deleted_permanent row survives its track's deletion.

Correct under either resolution of the dislike-lifecycle decision, so it
lands independently of it.

REVIEW-2026-07-30.md finding 6 (cascade only).

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

backend/src/services/db.service.ts

@@ -631,6 +631,48 @@ const MIGRATIONS: { id: string; sql: string }[] = [
       END $mig$;
     `,
   },
+  {
+    id: '20260730_feedback_track_id_set_null',
+    sql: `
+      -- feedback is an audit log, but feedback.track_id was
+      -- REFERENCES tracks(id) ON DELETE CASCADE. hardDeleteTrack() /
+      -- permanentlyDeleteTrack() insert a 'deleted_permanent' row and then
+      -- delete the track, so the audit row deleted itself — which is exactly
+      -- why the live feedback table contains zero 'deleted_permanent' rows.
+      -- Switch to ON DELETE SET NULL so audit rows outlive their track. Nothing
+      -- reads feedback.track_id expecting non-null (there are no SELECTs against
+      -- it at all; the only other reference is the dedup merge in
+      -- mergeTracks(), which rewrites track_id to the survivor).
+      ALTER TABLE feedback ALTER COLUMN track_id DROP NOT NULL;
+
+      DO $mig$
+      DECLARE
+        cname TEXT;
+      BEGIN
+        FOR cname IN
+          SELECT con.conname
+          FROM pg_constraint con
+          WHERE con.conrelid = 'feedback'::regclass
+            AND con.contype = 'f'
+            AND con.confrelid = 'tracks'::regclass
+            AND con.confdeltype <> 'n'   -- 'n' = SET NULL; anything else is wrong
+        LOOP
+          EXECUTE format('ALTER TABLE feedback DROP CONSTRAINT %I', cname);
+        END LOOP;
+
+        IF NOT EXISTS (
+          SELECT 1 FROM pg_constraint
+          WHERE conrelid = 'feedback'::regclass
+            AND contype = 'f'
+            AND confrelid = 'tracks'::regclass
+        ) THEN
+          ALTER TABLE feedback
+            ADD CONSTRAINT feedback_track_id_fkey
+            FOREIGN KEY (track_id) REFERENCES tracks(id) ON DELETE SET NULL;
+        END IF;
+      END $mig$;
+    `,
+  },
 ];
 
 export class DbService {