feat(reader): a Google Takeout export becomes watch state and Watch later

The YouTube API reads neither a user's watch history nor their Watch later list, so the Takeout
export is the only way in. `POST /api/takeout` takes one file of it (watch-history.json or
Watch later-videos.csv, told apart by content), keeps its videos in `takeout_items` and queues the
user's `takeout_import` run, whose outcome reaches their inbox.

The run marks watched the known videos the Shorts probe has cleared, at the Takeout time; a Short
is not marked, and a state the user set here (watched, hidden, or changed after that time) wins, by
the upsert's own condition. A video the catalog does not know yet waits with no outcome. Watch later
videos are fetched if unknown and appended oldest first.

An ad-hoc run can now ask for a completion notice (`run_ad_hoc(..., notify=True)`).

backend/app/reader/queries.py

@@ -1,5 +1,5 @@
-"""Reader's reads: a user's labels, saved views, watch state and playlists, for the HTTP layer and
-for the other units that ask about them.
+"""Reader's reads: a user's labels, saved views, watch state, playlists and Takeout import, for the HTTP
+layer and for the other units that ask about them.
 
 Every read of a user's row goes through `userscope`, so "someone else's row" is the same answer as
 a missing one. Reader reads Catalog's videos along its declared edge (a playlist's cover and
@@ -7,6 +7,8 @@ length, whether a video exists); another unit asks the questions at the end of t
 than joining Reader's tables itself.
 """
 
+from datetime import datetime
+
 from sqlalchemy import ColumnElement, and_, func, or_, select
 from sqlalchemy.orm import InstrumentedAttribute, Session, aliased
 
@@ -14,7 +16,16 @@ from app.collation import collated
 from app.feedquery import feed_columns, serialize
 from app.models import User
 from app.catalog.models import Channel, Subscription, Video
-from app.reader.models import ChannelTag, Playlist, PlaylistItem, SavedView, Tag, VideoState
+from app.reader.models import (
+    ChannelTag,
+    Playlist,
+    PlaylistItem,
+    SavedView,
+    Tag,
+    TakeoutItem,
+    TakeoutKind,
+    VideoState,
+)
 from app.userscope import owned_by, owned_row
 
 # --- catalog, read along the edge ------------------------------------------------------------
@@ -359,3 +370,55 @@ def user_tag_link(db: Session, user: User, channel_id: str, tag_id: int) -> Chan
             owned_by(ChannelTag.user_id, user),
         )
     ).scalar_one_or_none()
+
+
+# --- Google Takeout --------------------------------------------------------------------------
+
+
+def takeout_watched_ready(db: Session, user: User) -> list[tuple[int, str, datetime, bool]]:
+    """The user's waiting "watched" rows whose video the catalog knows and has probed for Shorts:
+    (row id, video id, Takeout time, is Short)."""
+    return list(
+        db.execute(
+            select(TakeoutItem.id, TakeoutItem.video_id, TakeoutItem.at, Video.is_short)
+            .join(Video, Video.id == TakeoutItem.video_id)
+            .where(
+                owned_by(TakeoutItem.user_id, user),
+                TakeoutItem.kind == TakeoutKind.WATCHED,
+                TakeoutItem.outcome.is_(None),
+                Video.shorts_probed,
+            )
+        ).tuples()
+    )
+
+
+def takeout_waiting(db: Session, user: User, kind: TakeoutKind) -> list[tuple[int, str]]:
+    """The user's rows of `kind` the import has not settled, oldest first: (row id, video id)."""
+    return list(
+        db.execute(
+            select(TakeoutItem.id, TakeoutItem.video_id)
+            .where(
+                owned_by(TakeoutItem.user_id, user),
+                TakeoutItem.kind == kind,
+                TakeoutItem.outcome.is_(None),
+            )
+            .order_by(TakeoutItem.at, TakeoutItem.id)
+        ).tuples()
+    )
+
+
+def known_video_ids(db: Session, ids: list[str]) -> set[str]:
+    """Which of `ids` the catalog holds."""
+    if not ids:
+        return set()
+    return set(db.execute(select(Video.id).where(Video.id.in_(ids))).scalars())
+
+
+def takeout_outcomes(db: Session, user: User) -> dict[tuple[str, str | None], int]:
+    """How many of the user's rows each kind and outcome holds (None = waiting)."""
+    rows = db.execute(
+        select(TakeoutItem.kind, TakeoutItem.outcome, func.count())
+        .where(owned_by(TakeoutItem.user_id, user))
+        .group_by(TakeoutItem.kind, TakeoutItem.outcome)
+    ).all()
+    return {(kind, outcome): n for kind, outcome, n in rows}