perf: concurrent worker with claim/execute split + graceful shutdown

- JobRepository::claim_next() — atomic SELECT FOR UPDATE SKIP LOCKED +
  UPDATE status=processing in one query, no duplicate claims
- ExecutePipelineHandler skips start() for already-claimed jobs
- Sweep spawns N concurrent tasks via JoinSet, claims are fast+sequential,
  execution is slow+concurrent
- Graceful shutdown: stop claiming, await all in-flight JoinSet tasks
- WORKER_CONCURRENCY env (default: CPU cores)
- DB_MAX_CONNECTIONS env (default: 20, was hardcoded 10)
- VolumeFileResolver impl for InMemoryFileStorage (test fix)

.env.example

@@ -39,6 +39,11 @@ STORAGE_PATH=./data/media
 # ============================================================================
 # MAX_UPLOAD_BYTES=268435456
 
+# ============================================================================
+# Worker concurrency (default: number of CPU cores)
+# ============================================================================
+# WORKER_CONCURRENCY=8
+
 # ============================================================================
 # Trash (default 30 days before permanent purge)
 # ============================================================================