refactor(downloads): introduce SourceAdapter — a thin per-source Strategy over yt-dlp

Consolidates source-specific policy (youtube / facebook / reddit / generic) that was scattered
across worker.py, backfill.py, service.py, formats.py and storage.py with no shared source
identity. app/downloads/adapters.py holds the base (= generic yt-dlp behaviour) + per-source
overrides; resolve_adapter(source_kind, source_ref, info) picks one by source_kind, the yt-dlp
extractor_key, or the URL host, falling back to generic. yt-dlp stays the engine — the adapter is
policy only.

Moved into adapters (behaviour-equivalent):
- uploader_url derivation (YouTube channel_url / Facebook uploader_id / reddit None) — removes the
  worker/backfill duplication (build_uploader_url + _should_refetch gone).
- the player-client fallback sets + retriable markers + YouTube extractor_args/POT (build_ydl_opts
  now takes source-agnostic opts plus an `extra` dict from the adapter).
- source_url reconstruction (service.source_url delegates to the adapter).

Two intentional (approved) behaviour changes:
- the player-client fallback is now YouTube-only; non-YouTube sources make a single extraction
  attempt instead of a pointless double attempt with youtube-only client args.
- the .nfo <uniqueid type=...> follows the source (adapter.nfo_uniqueid_type) instead of the
  hardcoded "youtube" that mislabelled every url-source download.

Deliberately left generic (documented non-moves): title cleaning (normalize_title is harmless
across sources), the classify marker table, thumbnail_to_jpg, the format selectors, and og.py's
YouTube-hqdefault og:image preference (a self-contained thumbnail heuristic on the share path).

Verified end-to-end on localdev across all four adapters: downloads succeed and uploader_url + the
.nfo uniqueid type are correct per source.

backend/tests/test_storage.py

@@ -137,3 +137,13 @@ class TestSafeAbsPath:
 
     def test_returns_none_for_a_nonexistent_file_inside_the_root(self, tmp_path: Path):
         assert safe_abs_path(str(tmp_path), "does/not/exist.mp4") is None
+
+
+def test_nfo_uniqueid_type_reflects_the_source():
+    # The .nfo <uniqueid type=...> now follows the source (via SourceAdapter.nfo_uniqueid_type),
+    # instead of the old hardcoded "youtube" that mislabelled every url-source download.
+    from app.downloads.storage import _nfo_xml
+
+    base = dict(video_id="v1", title="t", uploader="c", upload_date=None)
+    assert '<uniqueid type="youtube"' in _nfo_xml(MediaMeta(**base))  # default preserved
+    assert '<uniqueid type="facebook"' in _nfo_xml(MediaMeta(**base, uniqueid_type="facebook"))