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/app/downloads/formats.py

@@ -96,13 +96,13 @@ def build_ydl_opts(
     spec: dict,
     outtmpl: str,
     progress_hook,
-    pot_base_url: str | None = None,
-    player_clients: list[str] | None = None,
+    extra: dict | None = None,
 ) -> dict:
     """Assemble yt-dlp options for a single download to `outtmpl` (a full path template).
 
-    `pot_base_url` points the bgutil PO-token provider plugin at the sidecar server;
-    `player_clients` picks which YouTube clients to use (POT-capable ones like web use the token)."""
+    `extra` is source-specific options contributed by the SourceAdapter (e.g. YouTube's
+    `extractor_args` with the player-client set + bgutil PO-token base_url); it is merged onto the
+    generic options below. Everything here is source-agnostic."""
     s = normalize(spec)
     h = s["max_height"]
     opts: dict = {
@@ -127,16 +127,9 @@ def build_ydl_opts(
         "socket_timeout": 30,
     }
 
-    # Extractor args: which YouTube clients to use + where the bgutil PO-token sidecar lives
-    # (== --extractor-args "youtube:player_client=…;youtubepot-bgutilhttp:base_url=…"). Values
-    # are lists in the Python API.
-    ea: dict = {}
-    if player_clients:
-        ea["youtube"] = {"player_client": player_clients}
-    if pot_base_url:
-        ea["youtubepot-bgutilhttp"] = {"base_url": [pot_base_url]}
-    if ea:
-        opts["extractor_args"] = ea
+    # Source-specific options from the adapter (e.g. YouTube's extractor_args: player_client + POT).
+    if extra:
+        opts.update(extra)
 
     if s["mode"] == "a":
         opts["format"] = "bestaudio/best"