feat(channels): channel-explore backend — about metadata, ephemeral provenance, cleanup

Adds the backend for a dedicated channel page + ephemeral browsing of un-subscribed
channels:
- migration 0033: channels.{total_view_count,published_at,banner_url,external_links,
  from_explore}, videos.via_explore, explored_channels (per-user, grace-clocked).
- enrich channels with part=brandingSettings (banner/links) + statistics.viewCount +
  snippet.publishedAt — no extra quota.
- GET /api/channels/{id}: About detail + this user's relationship; lazy-enriches the new
  About fields (published_at sentinel).
- POST /api/channels/{id}/explore (require_human, quota-reserve guarded): records the
  exploration, flags the channel ephemeral unless followed, ingests one page of uploads
  (via_explore) + enriches; returns next_page_token for load-more.
- feed: per-explorer gate — a via_explore video is visible only to users who explored or
  subscribe to its channel, so exploration never leaks into everyone's catalog.
- subscribe = keep: clears from_explore + via_explore on the channel's videos.
- scheduler explore_cleanup job + explore_grace_days config: hard-delete explored-but-
  unkept channels and their untouched ephemeral videos after a grace period.

backend/app/youtube/client.py

@@ -187,7 +187,7 @@ class YouTubeClient:
             data = self._get(
                 "channels",
                 {
-                    "part": "snippet,contentDetails,statistics,topicDetails",
+                    "part": "snippet,contentDetails,statistics,topicDetails,brandingSettings",
                     "id": ",".join(batch),
                     "maxResults": 50,
                 },