feat(downloader): tier-limited classification, config defaults, yt-dlp support

- Classify Patreon "Not allowed to view post" warnings as TIER_LIMITED so
  subscription-gated runs are distinguished from genuine failures, and persist
  error_type/message on completed runs so the distinction survives to the UI.
- Fold PLATFORM_DEFAULTS into _get_default_config so gallery-dl.conf is a
  complete, editable document; _build_config_for_source now preserves the
  user's conf and only re-seeds missing platform sections.
- Add Reset-to-Defaults action in Settings (vs. Revert Changes which just
  reloads disk); show info banner when no gallery-dl.conf exists yet.
- Fix Discord embeds option: must be "all" string, not bool (gallery-dl
  iterates the value).
- Install yt-dlp + ffmpeg in Docker images so Patreon/Mux HLS video posts
  download instead of logging "Cannot import yt-dlp" and skipping.
- Recognize yt-dlp import failures as per-item errors so they don't mask
  TIER_LIMITED/NO_NEW_CONTENT classification.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/tasks/downloads.py

@@ -420,6 +420,13 @@ async def _download_source_async(source_id: int, download_id: int = None) -> dic
 
             if dl_result.success:
                 download.status = DownloadStatus.COMPLETED
+                # Some success states still carry an informational error_type
+                # (e.g., TIER_LIMITED — run completed, but the subscription tier
+                # blocked posts we'd have otherwise downloaded). Persist it so
+                # the UI can distinguish "fully succeeded" from "succeeded with
+                # external constraints".
+                download.error_type = dl_result.error_type.value if dl_result.error_type else None
+                download.error_message = dl_result.error_message
                 source.last_success = utcnow()
                 source.error_count = 0
                 logger.info(f"Download completed for {subscription_name}/{source_platform}: {dl_result.files_downloaded} files")