fix: every first watch state survives a write beside it, and a folded Watch later keeps its pin

A season marked watched, a pull from Plex and a YouTube resume point made their first row by a read
then an add, and failed on the unique key when a first checkpoint or the watched mark made it in
between. They go through the same insert-then-read as the checkpoint (the resume point is one
upsert); a pull that meets a local change still waiting for its push leaves it.

Folding duplicate Watch later lists moved their videos but not a Home pin, which the cascade then
deleted; the first pin of a dropped copy moves onto the kept list.

Marking one notice read trimmed before its own flag was flushed, so the trim left one past the cap.
The route test now crosses the cap and reads the result from a session of its own.

backend/tests/test_migration_0112.py

@@ -1,8 +1,8 @@
 """Migration 0112's data step, DB-lane: a user who already has two Watch later lists keeps the oldest,
-with the other's videos moved in after its own (one it already held is not added twice), and the
-one-per-user index then refuses a second list.
+with the other's videos moved in after its own (one it already held is not added twice) and a Home
+pin of the other moved onto it, and the one-per-user index then refuses a second list.
 
-The schema half is checked by test_migrations.py; a video lost on the way is invisible there.
+The schema half is checked by test_migrations.py; a video or a pin lost on the way is invisible there.
 """
 
 from datetime import datetime, timezone
@@ -17,6 +17,25 @@ from app.catalog.models import Channel, Video
 from app.db import engine
 
 
+def _lists(conn, user_id: int, n: int) -> list[int]:
+    values = ", ".join(f"(:u, 'Watch later', 'watch_later', {i})" for i in range(n))
+    return list(
+        conn.execute(
+            text(
+                f"INSERT INTO playlists (user_id, name, kind, position) VALUES {values} RETURNING id"
+            ),
+            {"u": user_id},
+        ).scalars()
+    )
+
+
+def _pin(conn, user_id: int, playlist_id: int, position: int) -> None:
+    conn.execute(
+        text("INSERT INTO comfort_pins (user_id, playlist_id, position) VALUES (:u, :p, :pos)"),
+        {"u": user_id, "p": playlist_id, "pos": position},
+    )
+
+
 def test_a_second_watch_later_is_folded_into_the_first(db, make_user):
     user, other = make_user(), make_user()
     db.add(Channel(id="UCm", title="M"))
@@ -28,15 +47,8 @@ def test_a_second_watch_later_is_folded_into_the_first(db, make_user):
     command.downgrade(cfg, "0111_audit_request_id")
     try:
         with engine.begin() as conn:
-            ids = conn.execute(
-                text(
-                    "INSERT INTO playlists (user_id, name, kind, position) VALUES"
-                    " (:u, 'Watch later', 'watch_later', 0), (:u, 'Watch later', 'watch_later', 1),"
-                    " (:o, 'Watch later', 'watch_later', 0) RETURNING id"
-                ),
-                {"u": user.id, "o": other.id},
-            ).scalars()
-            first, second, theirs = list(ids)
+            first, second, third = _lists(conn, user.id, 3)
+            theirs, their_second = _lists(conn, other.id, 2)
             conn.execute(
                 text(
                     "INSERT INTO playlist_items (playlist_id, video_id, position) VALUES"
@@ -44,6 +56,12 @@ def test_a_second_watch_later_is_folded_into_the_first(db, make_user):
                 ),
                 {"a": first, "b": second, "c": theirs},
             )
+            # The user pinned two of the copies, the kept one neither: the first on the shelf moves.
+            _pin(conn, user.id, third, 0)
+            _pin(conn, user.id, second, 1)
+            # The other pinned both: the kept one's pin stays, and is the only one.
+            _pin(conn, other.id, their_second, 0)
+            _pin(conn, other.id, theirs, 1)
     finally:
         command.upgrade(cfg, "head")
 
@@ -58,6 +76,10 @@ def test_a_second_watch_later_is_folded_into_the_first(db, make_user):
             )
         ).all()
         assert [tuple(r) for r in items] == [(first, "v1", 0), (first, "v2", 1), (theirs, "v3", 0)]
+        pins = conn.execute(
+            text("SELECT user_id, playlist_id, position FROM comfort_pins ORDER BY user_id")
+        ).all()
+        assert [tuple(r) for r in pins] == [(user.id, first, 0), (other.id, theirs, 1)]
     with pytest.raises(IntegrityError), engine.begin() as conn:
         conn.execute(
             text(