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/adapters/postgres/migrations/011_seed_default_plugins.sql

@@ -0,0 +1,25 @@
+-- Default plugins matching worker's InMemoryPluginRegistry
+INSERT INTO plugins (plugin_id, name, plugin_type, is_enabled, configuration)
+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, '{}')
+ON CONFLICT (plugin_id) DO NOTHING;
+
+-- Pipeline: extract_metadata → metadata_extractor
+INSERT INTO processing_pipelines (pipeline_id, trigger_event, steps)
+VALUES (
+    'b0000000-0000-4000-8000-000000000001',
+    'extract_metadata',
+    '[{"plugin_id": "a0000000-0000-4000-8000-000000000001", "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 (
+    'b0000000-0000-4000-8000-000000000002',
+    'sync_sidecar',
+    '[{"plugin_id": "a0000000-0000-4000-8000-000000000002", "step_order": 0, "configuration": {}}]'
+)
+ON CONFLICT (pipeline_id) DO NOTHING;