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/application/import_service.py

@@ -0,0 +1,96 @@
+"""LibraryImportService — imports files discovered by an indexable source.
+
+Batch sibling of :class:`UploadService`: for each discovered file it dedups on
+``(source, source_id)``, copies the file into managed storage, creates a minimal
+track (artist ``Unknown Artist``, ``metadata_status=pending``), and leaves the
+rest to enrichment (plan §6.2). Per-file failures are isolated — one bad file
+must not abort the whole scan (graceful degradation).
+"""
+
+import contextlib
+import uuid
+from dataclasses import dataclass
+
+from app.core.logging import get_logger
+from app.domain.ports import ArtistRepository, FileStorage, IndexableSource, TrackRepository
+from app.domain.sources import SourceFile
+
+log = get_logger(__name__)
+
+_UNKNOWN_ARTIST = "Unknown Artist"
+
+
+@dataclass(frozen=True)
+class ImportSummary:
+    source: str
+    seen: int
+    imported: int
+    skipped: int
+    failed: int
+
+
+class LibraryImportService:
+    def __init__(
+        self,
+        *,
+        tracks: TrackRepository,
+        artists: ArtistRepository,
+        storage: FileStorage,
+    ) -> None:
+        self._tracks = tracks
+        self._artists = artists
+        self._storage = storage
+
+    async def scan_and_import(
+        self, source: IndexableSource, *, added_by: uuid.UUID | None
+    ) -> ImportSummary:
+        seen = imported = skipped = failed = 0
+        for file in source.scan():
+            seen += 1
+            try:
+                existing = await self._tracks.get_by_source(source.name, file.source_id)
+                if existing is not None:
+                    skipped += 1
+                    continue
+                await self._import_one(source.name, file, added_by)
+                imported += 1
+            except Exception:
+                failed += 1
+                log.warning("import_file_failed", source=source.name, source_id=file.source_id)
+        summary = ImportSummary(
+            source=source.name, seen=seen, imported=imported, skipped=skipped, failed=failed
+        )
+        log.info(
+            "import_complete",
+            source=summary.source,
+            seen=summary.seen,
+            imported=summary.imported,
+            skipped=summary.skipped,
+            failed=summary.failed,
+        )
+        return summary
+
+    async def _import_one(
+        self, source_name: str, file: SourceFile, added_by: uuid.UUID | None
+    ) -> None:
+        track_id = uuid.uuid4()
+        key = f"tracks/{str(track_id)[:2]}/{track_id}.{file.file_format}"
+        await self._storage.save_file(key, file.path)
+        try:
+            artist = await self._artists.get_or_create(_UNKNOWN_ARTIST)
+            await self._tracks.add(
+                id=track_id,
+                title=file.suggested_title,
+                artist_id=artist.id,
+                storage_uri=key,
+                file_format=file.file_format,
+                file_size=file.file_size,
+                source=source_name,
+                source_id=file.source_id,
+                metadata_status="pending",
+                added_by=added_by,
+            )
+        except Exception:
+            with contextlib.suppress(Exception):
+                await self._storage.delete(key)
+            raise