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/app/notifications.py

@@ -49,7 +49,9 @@ def create_notification(
 
 def trim_read(db: Session, user_id: int, cap: int = READ_SOFT_CAP) -> int:
     """Delete a user's oldest READ notifications beyond `cap`. Returns the number removed.
-    Idempotent and cheap; safe to call after marking things read. Flushes; the caller commits."""
+    Idempotent and cheap; safe to call after marking things read. Flushes first, so a notice the
+    caller has just marked read counts; the caller commits."""
+    db.flush()
     ids = (
         db.execute(
             select(Notification.id)