Adds l4d2web/services/steam_workshop.py: parse_workshop_input (single ID,
URL, or multi-line batch), resolve_collection (HTTPS POST to
GetCollectionDetails), fetch_metadata_batch (HTTPS POST to
GetPublishedFileDetails with consumer_app_id == 550 enforcement that
raises WorkshopValidationError in add-mode and silently skips in
refresh-mode), download_to_cache (atomic + idempotent on mtime+size),
and refresh_all (ThreadPoolExecutor with per-item error collection).
Adds requests as an explicit dependency.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
