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)

crates/domain/src/processing/ports.rs

@@ -10,6 +10,7 @@ use std::sync::Arc;
 pub trait JobRepository: Send + Sync {
     async fn find_by_id(&self, id: &SystemId) -> Result<Option<Job>, DomainError>;
     async fn find_next_queued(&self) -> Result<Option<Job>, DomainError>;
+    async fn claim_next(&self) -> Result<Option<Job>, DomainError>;
     async fn find_by_batch(&self, batch_id: &SystemId) -> Result<Vec<Job>, DomainError>;
     async fn find_all(
         &self,