Par2 merge: a set held by two merged releases; one failed set doesn't stop a round

A par2 index and its repost both hold a set: moving both rows to the target
broke par2_sets' primary key, and the round failed on it every minute from
20:28 UTC (no merges at all). Duplicates among the others go first; a set that
fails is logged and the round goes on.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/services/par2_merger.ts

@@ -72,7 +72,11 @@ export class Par2Merger {
     const round: MergeRound = { sets: rows.length, merges: 0, releases: 0 }
     for (const { set_id: setId } of rows) {
       if (this.stopped) return round
-      const merged = await this.mergeSet(setId.trim())
+      // One set that fails doesn't hold up the others (nor the next round, which would start from the same point).
+      const merged = await this.mergeSet(setId.trim()).catch((error) => {
+        logger.error(`Par2 merge of set ${setId.trim()} failed: ${error.message}`)
+        return 0
+      })
       if (merged > 0) {
         round.merges++
         round.releases += merged + 1
@@ -129,8 +133,14 @@ export class Par2Merger {
       for (const table of ['release_chunks', 'probe_candidates', 'release_inspections', 'file_hashes']) {
         await trx.rawQuery(`UPDATE ${table} SET release_id = ? WHERE release_id = ANY(?)`, [target.id, others])
       }
-      // A set held by the target already isn't held twice.
-      await trx.rawQuery(`DELETE FROM par2_sets WHERE release_id = ANY(?) AND set_id IN (SELECT set_id FROM par2_sets WHERE release_id = ?)`, [others, target.id])
+      // A set held by the target already, or by two of the others (a par2 and its repost), isn't held twice (every
+      // round failed on the primary key from 20:28 UTC on 2026-10-12; LOG.md 2026-10-12 (30)).
+      await trx.rawQuery(
+        `DELETE FROM par2_sets p WHERE p.release_id = ANY(?)
+           AND (EXISTS (SELECT 1 FROM par2_sets t WHERE t.set_id = p.set_id AND t.release_id = ?)
+             OR EXISTS (SELECT 1 FROM par2_sets o WHERE o.set_id = p.set_id AND o.release_id = ANY(?) AND o.release_id < p.release_id))`,
+        [others, target.id, others]
+      )
       await trx.rawQuery(`UPDATE par2_sets SET release_id = ? WHERE release_id = ANY(?)`, [target.id, others])
       await trx.rawQuery(
         `UPDATE releases t SET