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/infrastructure/sources/registry.py

@@ -0,0 +1,41 @@
+"""Source registry — selection + enumeration of configured backends.
+
+Built from settings at the composition root. Only sources that are configured
+are registered (e.g. ``local`` appears only when ``LOCAL_MEDIA_IMPORT_PATH`` is
+set), so enumeration reflects what the instance can actually use.
+"""
+
+from typing import cast
+
+from app.core.config import Settings
+from app.domain.errors import NotFoundError, ValidationError
+from app.domain.ports import IndexableSource, SourceBackend
+from app.domain.sources import SourceInfo
+from app.infrastructure.sources.local_folder import LocalFolderSource
+
+
+class SourceRegistry:
+    def __init__(self, backends: list[SourceBackend]) -> None:
+        self._by_name = {backend.name: backend for backend in backends}
+
+    def get(self, name: str) -> SourceBackend:
+        backend = self._by_name.get(name)
+        if backend is None:
+            raise NotFoundError(f"Source {name!r} is not configured.")
+        return backend
+
+    def indexable(self, name: str) -> IndexableSource:
+        backend = self.get(name)
+        if not hasattr(backend, "scan"):
+            raise ValidationError(f"Source {name!r} cannot be indexed.")
+        return cast(IndexableSource, backend)
+
+    def infos(self) -> list[SourceInfo]:
+        return [backend.info() for backend in self._by_name.values()]
+
+
+def build_source_registry(settings: Settings) -> SourceRegistry:
+    backends: list[SourceBackend] = []
+    if settings.local_media_import_path is not None:
+        backends.append(LocalFolderSource(settings.local_media_import_path))
+    return SourceRegistry(backends)