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/routes/feed.py

@@ -16,6 +16,7 @@ from app.db import get_db
 from app.models import (
     Channel,
     ChannelTag,
+    ExploredChannel,
     Playlist,
     PlaylistItem,
     SearchFind,
@@ -180,6 +181,23 @@ def _filtered_query(
     # either way they shouldn't show in any feed view meanwhile.
     query = query.where(Video.unavailable_since.is_(None))
 
+    # Explored videos are PER-EXPLORER private: a via_explore video (ingested when someone
+    # opened an un-subscribed channel's page) is visible only to users who explored or
+    # subscribe to its channel, so one user's curiosity never leaks into everyone else's
+    # catalog. Applied in BOTH scopes; the channel page reaches its ephemeral videos by
+    # asking for scope=all + channel_id (the channel is in this accessible set).
+    accessible_explore = (
+        select(Subscription.channel_id).where(Subscription.user_id == user.id)
+    ).union(
+        select(ExploredChannel.channel_id).where(ExploredChannel.user_id == user.id)
+    )
+    query = query.where(
+        or_(
+            Video.via_explore.is_(False),
+            Video.channel_id.in_(accessible_explore),
+        )
+    )
+
     # Source filter. In the shared Library ("all" scope) it uses the GLOBAL via_search flag
     # (search-discovered by anyone). In "my" scope it's the user's OWN provenance:
     # "organic" = your subscriptions, "search" = videos you found via your own search,