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: "..." } }

crates/worker/Cargo.toml

@@ -19,6 +19,8 @@ adapters-exif      = { workspace = true }
 adapters-thumbnail = { workspace = true }
 adapters-sidecar   = { workspace = true }
 async-nats         = { workspace = true }
+sha2               = { workspace = true }
+uuid               = { workspace = true }
 
 futures           = { workspace = true }
 tokio             = { workspace = true }