feat(reader): the channels a Takeout history shows watched but not followed

The import asks YouTube how long the watched videos of the candidate
channels are (five or more watched, none the user follows, blocked, put
out or owns), keeps the length on the Takeout row and names the channels
that make the list: five videos of five minutes or more. The shorts job
asks what an import could not. A channel's counted videos come in as an
exploration's when its list is opened, ready to play; a channel can be put
out of the list and back.

backend/app/reader/models.py

@@ -254,8 +254,9 @@ class TakeoutItem(Base, TimestampMixin):
     """One video from a user's Google Takeout export: watched (with when) or in their YouTube Watch
     later list (with when it was added). The rows are both the import job's input and its memory: a
     video the catalog does not know yet waits here, with no outcome, until it enters the catalog.
-    The id is not a foreign key for that reason. The channel is kept for a later "channels you
-    watch but do not follow" list."""
+    The id is not a foreign key for that reason. A watched row's channel and length feed the
+    "channels you watch but do not follow" list: the length is asked of YouTube for that list alone
+    and kept here, so its videos need not enter the shared catalog to be counted."""
 
     __tablename__ = "takeout_items"
     __table_args__ = (
@@ -277,3 +278,21 @@ class TakeoutItem(Base, TimestampMixin):
     # Watched: the last time in the export; Watch later: when it was added.
     at: Mapped[datetime] = mapped_column(DateTime(timezone=True))
     outcome: Mapped[str | None] = mapped_column(String(12))
+    # When YouTube was asked for the video's length (NULL = not yet), and the length it gave; a
+    # video it no longer has keeps a NULL length.
+    looked_up_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    duration_seconds: Mapped[int | None] = mapped_column(Integer)
+
+
+class TakeoutChannelDismissal(Base, TimestampMixin):
+    """A channel the user put out of their "watched but not followed" list. The id is not a foreign
+    key: the channel may never have entered the catalog, or may leave it, and the choice stays."""
+
+    __tablename__ = "takeout_channel_dismissals"
+    __table_args__ = (
+        UniqueConstraint("user_id", "channel_id", name="uq_takeout_channel_dismissal"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"))
+    channel_id: Mapped[str] = mapped_column(String(32))