feat: seed default plugins/pipelines, auto-enqueue jobs on asset ingest

- Migration seeds metadata_extractor, sidecar_sync, no_op plugins
- Pipelines: extract_metadata → metadata_extractor, sync_sidecar → sidecar_sync
- Worker reacts to AssetIngested → enqueues ExtractMetadata job
- Worker reacts to SidecarSyncRequested → enqueues SyncSidecar job
- Closes the ingest-to-processing loop end-to-end

crates/worker/src/factories/processing.rs

@@ -1,4 +1,4 @@
-use application::processing::{ExecutePipelineHandler, ProcessNextJobHandler};
+use application::processing::{EnqueueJobHandler, ExecutePipelineHandler, ProcessNextJobHandler};
 use domain::ports::{EventPublisher, PluginRegistry};
 use std::sync::Arc;
 
@@ -20,3 +20,10 @@ pub fn build_process_next_handler(
 
     ProcessNextJobHandler::new(repos.job.clone(), execute_pipeline)
 }
+
+pub fn build_enqueue_handler(
+    repos: &Repos,
+    event_pub: Arc<dyn EventPublisher>,
+) -> EnqueueJobHandler {
+    EnqueueJobHandler::new(repos.job.clone(), event_pub)
+}