files partitions: no deadlock, four ahead, and a failure no longer stops the GC sweep

Creating a partition deadlocked with file inserts 11 times in an hour, and
each failure ended the GC sweep before its deletes. The parent is now
locked first (lock_timeout 10 s), four partitions are kept ahead, and an
error is logged while the sweep goes on.

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

tests/functional/file_partitions.spec.ts

@@ -39,10 +39,11 @@ test.group('files partitions', (group) => {
       )
       return Number(rows[0].upper)
     }
+    await FilePartitions.ensure()
     const before = await upper()
     assert.deepEqual(await FilePartitions.ensure(), [])
-    // A file near the end of the covered ids: one more partition is needed.
-    await db.rawQuery(`SELECT setval('files_id_seq', ?, false)`, [before - FILE_PARTITION_SIZE - 10])
+    // Ids handed out up to just inside the four partitions kept ahead: one more is needed.
+    await db.rawQuery(`SELECT setval('files_id_seq', ?, false)`, [before - 4 * FILE_PARTITION_SIZE + 10])
     await FileStore.insert([file('e'.repeat(40), 'late.bin')])
     assert.deepEqual(await FilePartitions.ensure(), [`files_p${before}`])
     assert.equal(await upper(), before + FILE_PARTITION_SIZE)