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>

commands/GcScheduler.ts

@@ -79,8 +79,15 @@ export default class GcScheduler extends BaseCommand {
       }
     }
 
-    // Inserts fail without a partition for their id (D39).
-    if (!this.dryRun) await FilePartitions.ensure()
+    // Inserts fail without a partition for their id (D39). Four are kept ahead, so a failure here
+    // can wait for the next sweep; it used to end the sweep, deletes and all (LOG.md 2026-10-07 (4)).
+    if (!this.dryRun) {
+      try {
+        await FilePartitions.ensure()
+      } catch (err: any) {
+        this.logger.error(`files partitions: ${err.message}; trying again next sweep.`)
+      }
+    }
 
     const trimmed = await this.trimBodyQueue()
     if (trimmed) this.logger.info(`Body queue: ${this.verb('drop', 'dropped')} ${trimmed} oldest waiting jobs.`)