fix(player): persist watched at end-of-playback (atomic upsert)

The in-app player marks a video watched when it reaches the end, firing
POST /videos/{id}/state at the same instant as a progress checkpoint. The
progress endpoint deletes the 'new' video_states row near the end, so the
concurrent state UPDATE matched 0 rows and raised StaleDataError -> 500. The
watched status was never persisted (and Feed swallowed the error), so the
video stayed in the unwatched feed even after a refresh.

- set_video_state: atomic INSERT ... ON CONFLICT (uq_user_video) DO UPDATE for
  watched/hidden, immune to a concurrent delete; tolerate StaleDataError on the
  'new' branch and in the progress endpoint.
- PlayerModal: skip the redundant near-end progress checkpoint when we just
  auto-marked watched, removing the self-inflicted race.
- Feed: drop the optimistic override if the server rejects the change, so a
  failed request can't leave a card phantom-hidden.

frontend/src/components/Feed.tsx

@@ -147,7 +147,15 @@ export default function Feed({
       api
         .setState(id, status)
         .then(() => qc.invalidateQueries({ queryKey: ["feed"] }))
-        .catch(() => {});
+        .catch(() =>
+          // Server rejected the change — drop the optimistic override so the card
+          // reverts to the real (unchanged) state instead of staying phantom-hidden.
+          setOverrides((o) => {
+            const next = { ...o };
+            delete next[id];
+            return next;
+          })
+        );
       if (status === "hidden") {
         const v = loadedRef.current.find((x) => x.id === id);
         notify({