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/models.py

@@ -4,6 +4,7 @@ and the comfort shelf on Home. Only this unit writes them; another unit reads th
 edge, or asks this unit's service."""
 
 from datetime import datetime
+from enum import StrEnum
 
 from sqlalchemy import (
     Boolean,
@@ -213,3 +214,51 @@ class ComfortPin(Base, TimestampMixin):
     # 0-based shelf order. The shelf is short and hand-curated, so a dense integer the reorder
     # endpoint rewrites wholesale beats a fractional index nobody would ever exercise here.
     position: Mapped[int] = mapped_column(Integer, default=0, server_default="0", nullable=False)
+
+
+class TakeoutKind(StrEnum):
+    """What a Google Takeout row says the user did with a video."""
+
+    WATCHED = "watched"  # history/watch-history.json
+    WATCH_LATER = "watch_later"  # playlists/Watch later-videos.csv
+
+
+class TakeoutOutcome(StrEnum):
+    """What the import did with a row; NULL = still waiting for its video to be known (and, for a
+    watched one, probed for Shorts)."""
+
+    WATCHED = "watched"  # the video is marked watched at the Takeout time
+    KEPT = "kept"  # the state the user set here (watched, hidden, or changed later) stays
+    SHORT = "short"  # a Short: swiping one past is not watching it
+    ADDED = "added"  # added to Watch later
+    PRESENT = "present"  # already in Watch later
+    MISSING = "missing"  # YouTube no longer has the video
+
+
+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."""
+
+    __tablename__ = "takeout_items"
+    __table_args__ = (
+        UniqueConstraint("user_id", "kind", "video_id", name="uq_takeout_item"),
+        CheckConstraint("kind IN ('watched', 'watch_later')", name="ck_takeout_item_kind"),
+        # The waiting rows, by video: what a newly known video looks up.
+        Index(
+            "ix_takeout_items_waiting",
+            "video_id",
+            postgresql_where=text("outcome IS NULL"),
+        ),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"))
+    kind: Mapped[str] = mapped_column(String(12))
+    video_id: Mapped[str] = mapped_column(String(16))
+    channel_id: Mapped[str | None] = mapped_column(String(32))
+    # 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))