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/worker/src/bootstrap.rs

@@ -1,16 +1,16 @@
 use std::sync::Arc;
 
 use application::catalog::DeleteAssetHandler;
-use application::processing::{EnqueueJobHandler, ProcessNextJobHandler};
+use application::processing::{EnqueueJobHandler, ExecutePipelineHandler};
 use domain::ports::{AssetRepository, JobRepository};
 
 use crate::config::WorkerConfig;
 use crate::factories::{
-    Repos, build_enqueue_handler, build_plugin_registry, build_process_next_handler,
+    Repos, build_enqueue_handler, build_executor, build_plugin_registry,
 };
 
 pub struct WorkerServices {
-    pub process_next: Arc<ProcessNextJobHandler>,
+    pub executor: Arc<ExecutePipelineHandler>,
     pub enqueue: Arc<EnqueueJobHandler>,
     pub job_repo: Arc<dyn JobRepository>,
     pub asset_repo: Arc<dyn AssetRepository>,
@@ -57,11 +57,7 @@ pub async fn build(config: &WorkerConfig) -> anyhow::Result<WorkerServices> {
         event_pub.clone(),
     ));
 
-    let process_next = Arc::new(build_process_next_handler(
-        &repos,
-        registry,
-        event_pub.clone(),
-    ));
+    let executor = Arc::new(build_executor(&repos, registry, event_pub.clone()));
     let job_repo: Arc<dyn JobRepository> = repos.job.clone();
     let asset_repo: Arc<dyn AssetRepository> = repos.asset.clone();
     let enqueue = Arc::new(build_enqueue_handler(&repos, event_pub.clone()));
@@ -80,7 +76,7 @@ pub async fn build(config: &WorkerConfig) -> anyhow::Result<WorkerServices> {
     let event_consumer = adapters_event_transport::EventConsumerAdapter::new(consumer_source);
 
     Ok(WorkerServices {
-        process_next,
+        executor,
         enqueue,
         job_repo,
         asset_repo,