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>

.env.example

@@ -27,6 +27,10 @@ MEDIA_PATH=/data/media
 TRANSCODE_CACHE_PATH=/data/transcode-cache
 MAX_PARALLEL_DOWNLOADS=2
 
+# sources — mounted folder the `local` source indexes (copies into MEDIA_PATH).
+# Unset → the local source is not registered. Mount read-only in compose.
+# LOCAL_MEDIA_IMPORT_PATH=/import
+
 # external services (all optional — backend degrades gracefully if unset)
 # ML_SERVICE_URL=http://ml:9000
 # ACOUSTID_API_KEY=