Rebuild: a release key taken by another group's writer gets the next number

Writers lock their group only, so two groups' batches can number the
same identity key at once (a spam post crossposted to 67 groups:
duplicate key ...:65, HTTP 500, the batch retried). Releases are now
inserted with ON CONFLICT (release_key) DO NOTHING, and those skipped
take the next number (up to 20 tries).

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

tests/functional/batch_store.spec.ts

@@ -127,4 +127,15 @@ test.group('BatchStore', (group) => {
     assert.lengthOf(rows, 2)
     assert.equal(rows[0].identity_key, rows[1].identity_key)
   })
+
+  test('a release key taken meanwhile by another group\'s writer is skipped for the next number', async ({ assert }) => {
+    await storeIn(GROUP, 1, upload(10, 'Some.Show.S01E01.1080p.mkv', 2, 'Some.Show.S01E01.1080p [1/1]', '2026-10-05T10:00:00Z'))
+    const [first] = await releasesOf()
+    // As if another group's batch numbered the key at the same moment: key:2 exists, not counted for this key.
+    const [g] = await db.table('groups').insert({ name: 'alt.binaries.third', priority: 3 }).returning('id')
+    await db.table('releases').insert({ release_key: `${first.identity_key}:2`, identity_key: 'someone-else', group_id: g.id, name: 'x', source: 'subject', poster: 'p', groups: JSON.stringify([]), files: 0, segments: 0, bytes: 0, chunks: 0 })
+    await storeIn('alt.binaries.other', 1, upload(10, 'Some.Show.S01E01.1080p.mkv', 2, 'Some.Show.S01E01.1080p [1/1]', '2026-10-05T10:01:00Z', 'alt.binaries.other'))
+    const keys = (await releasesOf()).filter((r) => r.identity_key === first.identity_key).map((r) => r.release_key)
+    assert.deepEqual(keys, [first.identity_key, `${first.identity_key}:3`])
+  })
 })