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_formats.py

@@ -9,6 +9,15 @@ def _opts(spec):
     return formats.build_ydl_opts(spec, "/tmp/%(id)s.%(ext)s", lambda d: None)
 
 
+def test_adapter_extra_opts_are_merged_in():
+    # The SourceAdapter contributes source-specific opts (e.g. YouTube's extractor_args) via `extra`.
+    extra = {"extractor_args": {"youtube": {"player_client": ["web"]}}}
+    opts = formats.build_ydl_opts({}, "/tmp/%(id)s.%(ext)s", lambda d: None, extra=extra)
+    assert opts["extractor_args"]["youtube"]["player_client"] == ["web"]
+    # No extra → no extractor_args (a generic source contributes nothing).
+    assert "extractor_args" not in formats.build_ydl_opts({}, "/tmp/x", lambda d: None)
+
+
 def test_mp4_default_selects_avc1_with_progressive_fallback():
     fmt = _opts({"mode": "av", "container": "mp4"})["format"]
     # avc1 video-only first, then the avc1 progressive, then an unrestricted fallback.