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/sync/subscriptions.py

@@ -18,6 +18,29 @@ def _to_int(value) -> int | None:
         return None
 
 
+def _parse_dt(value: str | None) -> datetime | None:
+    if not value:
+        return None
+    try:
+        return datetime.fromisoformat(value.replace("Z", "+00:00"))
+    except ValueError:
+        return None
+
+
+def _external_links(branding: dict) -> list | None:
+    """Channel "About" links. The API exposes a few in brandingSettings.channel.profileColor/
+    /... varies; the durable spot is brandingSettings.channel.links is gone post-2023, so we
+    read what's present (settings/links) defensively and keep [{title, url}] pairs."""
+    channel = branding.get("channel", {})
+    links = channel.get("links") or []
+    out = [
+        {"title": l.get("title"), "url": l.get("url")}
+        for l in links
+        if isinstance(l, dict) and l.get("url")
+    ]
+    return out or None
+
+
 def apply_channel_details(db: Session, items: list[dict]) -> None:
     now = datetime.now(timezone.utc)
     for item in items:
@@ -28,6 +51,7 @@ def apply_channel_details(db: Session, items: list[dict]) -> None:
         content = item.get("contentDetails", {})
         stats = item.get("statistics", {})
         topics = item.get("topicDetails", {})
+        branding = item.get("brandingSettings", {})
 
         channel.title = snippet.get("title") or channel.title
         channel.handle = snippet.get("customUrl")
@@ -40,6 +64,10 @@ def apply_channel_details(db: Session, items: list[dict]) -> None:
         channel.uploads_playlist_id = content.get("relatedPlaylists", {}).get("uploads")
         channel.subscriber_count = _to_int(stats.get("subscriberCount"))
         channel.video_count = _to_int(stats.get("videoCount"))
+        channel.total_view_count = _to_int(stats.get("viewCount"))
+        channel.published_at = _parse_dt(snippet.get("publishedAt"))
+        channel.banner_url = branding.get("image", {}).get("bannerExternalUrl")
+        channel.external_links = _external_links(branding)
         channel.topic_categories = topics.get("topicCategories")
         channel.details_synced_at = now