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/adapters/postgres/migrations/011_seed_default_plugins.sql

@@ -4,7 +4,8 @@ VALUES
     ('a0000000-0000-4000-8000-000000000001', 'metadata_extractor',  'media_processor', true, '{}'),
     ('a0000000-0000-4000-8000-000000000002', 'sidecar_sync',        'sidecar_writer',  true, '{}'),
     ('a0000000-0000-4000-8000-000000000003', 'no_op',               'scheduled_task',   true, '{}'),
-    ('a0000000-0000-4000-8000-000000000004', 'thumbnail_generator', 'media_processor', true, '{}')
+    ('a0000000-0000-4000-8000-000000000004', 'thumbnail_generator',  'media_processor', true, '{}'),
+    ('a0000000-0000-4000-8000-000000000005', 'directory_scanner',    'media_processor', true, '{}')
 ON CONFLICT (plugin_id) DO NOTHING;
 
 -- Pipeline: extract_metadata → metadata_extractor, then thumbnail_generator
@@ -26,6 +27,15 @@ VALUES (
 )
 ON CONFLICT (pipeline_id) DO NOTHING;
 
+-- Pipeline: scan_directory → directory_scanner
+INSERT INTO processing_pipelines (pipeline_id, trigger_event, steps)
+VALUES (
+    'b0000000-0000-4000-8000-000000000004',
+    'scan_directory',
+    '[{"plugin_id": "a0000000-0000-4000-8000-000000000005", "step_order": 0, "configuration": {}}]'
+)
+ON CONFLICT (pipeline_id) DO NOTHING;
+
 -- Pipeline: sync_sidecar → sidecar_sync
 INSERT INTO processing_pipelines (pipeline_id, trigger_event, steps)
 VALUES (