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/application/src/processing/commands/execute_pipeline.rs

@@ -60,8 +60,10 @@ impl ExecutePipelineHandler {
             .await?
             .ok_or_else(|| DomainError::NotFound(format!("Job {} not found", cmd.job_id)))?;
 
-        job.start()?;
-        self.job_repo.save(&job).await?;
+        if job.status == domain::entities::JobStatus::Queued {
+            job.start()?;
+            self.job_repo.save(&job).await?;
+        }
 
         let trigger = job_type_to_trigger(&job.job_type);
         let pipelines = self.pipeline_repo.find_by_trigger(trigger).await?;