feat(sources): local_folder source backend + import pipeline

First ingest path beyond manual upload (plan §1C). Source abstraction +
the first concrete backend, so a homelab can index an existing library.

- domain: SourceBackend/IndexableSource ports + SourceInfo/SourceFile shapes
- infrastructure/sources: LocalFolderSource (walks a mounted dir, idempotent
  source_id = relative path) + registry built from settings
- application: LibraryImportService — batch sibling of UploadService; dedup on
  (source, source_id), copy into storage, minimal track (metadata_status=pending,
  enrichment fills the rest in 1D), per-file failures isolated
- workers: scan_local_folder arq task (registered) + enqueue helper (503 if
  Redis down)
- api: GET /sources, POST /sources/{source}/scan (admin, enqueues), /health
- config: LOCAL_MEDIA_IMPORT_PATH; README + .env.example documented
- tests: scanner, registry, import service (fakes) + DB-gated sources API path

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/workers/arq_worker.py

@@ -10,6 +10,7 @@ from arq.connections import RedisSettings
 
 from app.core.config import get_settings
 from app.core.logging import configure_logging, get_logger
+from app.workers.tasks.import_task import scan_local_folder
 
 log = get_logger("worker")
 
@@ -24,12 +25,8 @@ async def shutdown(_ctx: dict[str, Any]) -> None:
     log.info("worker_shutdown")
 
 
-async def _noop(_ctx: dict[str, Any]) -> None:
-    pass
-
-
 class WorkerSettings:
-    functions: ClassVar[list[Any]] = [_noop]  # populated as tasks are implemented
+    functions: ClassVar[list[Any]] = [scan_local_folder]
     on_startup = startup
     on_shutdown = shutdown
     max_jobs = get_settings().max_parallel_downloads