executor-v1.110
DAH-2391: isolate metrics collection from the executor event loop (PR #1130).
Metrics collection now runs on a dedicated ThreadPoolExecutor with a per-call 8s timeout and a concurrency semaphore (immediate 503 on overload); df runs via a killable subprocess with a 5s timeout. A hung FUSE mount or a burst of renter polling can no longer block /ping or /upload_ssh_key. No other executor-code changes since executor-v1.109.