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/tasks/import_task.py

@@ -0,0 +1,46 @@
+"""arq task: scan an indexable source and import its files into the library.
+
+Heavy work (directory walk + file copies) belongs off the request cycle
+(CLAUDE.md). The HTTP endpoint enqueues this; the worker runs it with its own
+transactional session.
+"""
+
+import uuid
+from typing import Any
+
+from app.application.import_service import LibraryImportService
+from app.core.config import get_settings
+from app.core.logging import get_logger
+from app.infrastructure.db import session_scope
+from app.infrastructure.db.repositories import (
+    SqlAlchemyArtistRepository,
+    SqlAlchemyTrackRepository,
+)
+from app.infrastructure.sources.registry import build_source_registry
+from app.infrastructure.storage.provider import get_file_storage
+
+log = get_logger("worker.import")
+
+
+async def scan_local_folder(
+    _ctx: dict[str, Any], *, source: str = "local", added_by: str | None = None
+) -> dict[str, Any]:
+    registry = build_source_registry(get_settings())
+    backend = registry.indexable(source)
+    actor = uuid.UUID(added_by) if added_by else None
+
+    async with session_scope() as session:
+        service = LibraryImportService(
+            tracks=SqlAlchemyTrackRepository(session),
+            artists=SqlAlchemyArtistRepository(session),
+            storage=get_file_storage(),
+        )
+        summary = await service.scan_and_import(backend, added_by=actor)
+
+    return {
+        "source": summary.source,
+        "seen": summary.seen,
+        "imported": summary.imported,
+        "skipped": summary.skipped,
+        "failed": summary.failed,
+    }