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/queue.py

@@ -0,0 +1,30 @@
+"""Enqueue helper — submit a job to the arq queue from the request cycle.
+
+A short-lived pool per call keeps things simple (enqueues are rare, admin-driven
+actions). Redis being down degrades to a clean 503 rather than a crash
+(graceful degradation)."""
+
+from typing import Any
+
+from arq import create_pool
+from arq.connections import RedisSettings
+
+from app.core.config import get_settings
+from app.domain.errors import DependencyUnavailableError
+
+
+async def enqueue(function: str, **kwargs: Any) -> str:
+    """Enqueue ``function`` by name, returning the job id. Raises
+    :class:`DependencyUnavailableError` if the queue can't be reached."""
+    settings = get_settings()
+    try:
+        pool = await create_pool(RedisSettings.from_dsn(str(settings.redis_url)))
+    except Exception as exc:
+        raise DependencyUnavailableError("Task queue (Redis) is unavailable.") from exc
+    try:
+        job = await pool.enqueue_job(function, **kwargs)
+    finally:
+        await pool.aclose()
+    if job is None:
+        raise DependencyUnavailableError("Could not enqueue job.")
+    return str(job.job_id)