docs(progress): document the intentional feed-vs-Plex finish-margin divergence

R12 S5 item: the feed's FINISH_MARGIN_SECONDS (10) and the Plex player's
_FINISH_MARGIN_S (30) were flagged as drifted duplicates. They are a
deliberate divergence, not drift — the feed clears a short video's resume
bar, Plex marks long-form items (movies/episodes with credits) watched.
Cross-reference + document both so it reads as intentional.

backend/app/routes/feed.py

@@ -41,6 +41,8 @@ VALID_STATES = {"new", "watched", "hidden"}
 # Resume-position thresholds (mirror the client): positions below this are "didn't
 # really start" and within this of the end are "basically finished" — neither is worth
 # storing, so we clear the position instead (the video shows no progress bar).
+# 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