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/models.py

@@ -165,6 +165,12 @@ class Channel(Base, TimestampMixin):
     uploads_playlist_id: Mapped[str | None] = mapped_column(String(64))
     subscriber_count: Mapped[int | None] = mapped_column(BigInteger)
     video_count: Mapped[int | None] = mapped_column(BigInteger)
+    # Channel "About" metadata (fetched with the same channels.list call, no extra quota):
+    # lifetime view count, the channel's join date, banner image, and external links.
+    total_view_count: Mapped[int | None] = mapped_column(BigInteger)
+    published_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    banner_url: Mapped[str | None] = mapped_column(String(1024))
+    external_links: Mapped[list | None] = mapped_column(JSON)
     topic_categories: Mapped[list | None] = mapped_column(JSON)
     default_language: Mapped[str | None] = mapped_column(String(16))
     country: Mapped[str | None] = mapped_column(String(8))
@@ -183,6 +189,14 @@ class Channel(Base, TimestampMixin):
     from_search: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default="false", index=True
     )
+    # True when this channel exists only because a user EXPLORED it (opened its page without
+    # subscribing) — an ephemeral, per-explorer addition. The cleanup job hard-deletes such a
+    # channel (and its via_explore videos) once nobody keeps it (no subscription, no live
+    # ExploredChannel row, no per-user state) after a grace period. Cleared when someone
+    # subscribes (the "keep" signal promotes it to organic).
+    from_explore: Mapped[bool] = mapped_column(
+        Boolean, default=False, server_default="false", index=True
+    )
 
     videos: Mapped[list["Video"]] = relationship(back_populates="channel")
     subscriptions: Mapped[list["Subscription"]] = relationship(back_populates="channel")
@@ -260,6 +274,14 @@ class Video(Base, TimestampMixin):
     via_search: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default="false", index=True
     )
+    # True when this video entered the catalog because a user EXPLORED its (un-subscribed)
+    # channel. Like via_search it's hidden from the shared Library by default, but it's
+    # additionally PER-USER private: a via_explore video is only visible to users who
+    # explored or subscribe to its channel (a gate in the feed query), so one user's
+    # curiosity never floods everyone else's catalog. Left False by the organic ingest paths.
+    via_explore: Mapped[bool] = mapped_column(
+        Boolean, default=False, server_default="false", index=True
+    )
     # none | live | upcoming | premiere | was_live
     live_status: Mapped[str] = mapped_column(
         String(16), default="none", server_default="none", index=True
@@ -388,6 +410,31 @@ class SearchFind(Base, TimestampMixin):
     )
 
 
+class ExploredChannel(Base, TimestampMixin):
+    """Per-user record of a channel the user opened the page of WITHOUT subscribing.
+
+    Exploring an un-subscribed channel ingests its recent uploads (flagged via_explore) so the
+    user can browse them, but keeps them out of everyone else's catalog. This row (a) gates the
+    explorer's access to those ephemeral videos in the feed query and (b) drives the grace-period
+    purge: a from_explore channel with no live ExploredChannel row (and no subscription / no
+    per-user state on its videos) is hard-deleted by the cleanup job. Subscribing is the "keep"
+    signal — it promotes the channel to organic and these rows become irrelevant. `created_at`
+    (from TimestampMixin) is the grace clock; a re-visit refreshes it."""
+
+    __tablename__ = "explored_channels"
+    __table_args__ = (
+        UniqueConstraint("user_id", "channel_id", name="uq_user_explored_channel"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), index=True
+    )
+    channel_id: Mapped[str] = mapped_column(
+        ForeignKey("channels.id", ondelete="CASCADE"), index=True
+    )
+
+
 class ApiQuotaUsage(Base):
     """Tracks YouTube Data API units spent per Pacific-time day (the quota resets at
     midnight Pacific). The whole app shares one daily budget."""