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>

Dockerfile

@@ -18,6 +18,7 @@ RUN DEBIAN_FRONTEND=noninteractive apt-get update \
         gcc \
         libpq-dev \
         gosu \
+        ffmpeg \
     && rm -rf /var/lib/apt/lists/*
 
 # Install Python dependencies