test(player): hold the session stop and the kept-point rule to the server

- The client's resume thresholds equal the server's (read from both sources).
- The session stop's URL matches the route the server serves it on.
- The player stops its server session by its item and its own id on an unload and on close,
  and keeps it when the page only goes into the back/forward cache.

backend/app/routes/_common.py

@@ -23,9 +23,10 @@ M = TypeVar("M", bound=Base)
 # Resume-position thresholds of a player that only keeps a resume point (the YouTube player, the
 # library's): positions below this are "didn't really start" and within this of the end are
 # "basically finished" — neither is worth storing, so the position is cleared instead (no progress
-# bar). The client's local resume (`lib/resume.ts`) keeps the same two numbers. NB: the Plex player
-# intentionally uses a LARGER finish margin (30s) because it marks long-form items watched, not just
-# clears a resume bar — see routes/plex/playback.py.
+# bar). The client's local resume (`lib/resume.ts`) keeps the same two numbers, held equal by
+# `resume.contract.test.ts` beside it. NB: the Plex player intentionally uses a LARGER finish margin
+# (30s) because it marks long-form items watched, not just clears a resume bar — see
+# routes/plex/playback.py.
 PROGRESS_MIN_SECONDS = 5
 FINISH_MARGIN_SECONDS = 10