feat: directory scanner plugin — walk library paths, auto-register assets

- DirectoryScannerPlugin: recursive directory walk via FileStoragePort
- Computes SHA256 checksums, classifies media by extension
- Registers each file via RegisterAssetHandler (triggers AssetIngested → extract_metadata pipeline)
- Reads library_path_id from job payload, looks up volume + path
- Seeded plugin + scan_directory pipeline
- Trigger via POST /jobs with { job_type: "ScanDirectory", payload: { library_path_id: "..." } }

Cargo.lock

@@ -4364,9 +4364,11 @@ dependencies = [
  "domain",
  "dotenvy",
  "futures",
+ "sha2",
  "tokio",
  "tracing",
  "tracing-subscriber",
+ "uuid",
 ]
 
 [[package]]