fix(career): hours polish — recency stamp + non-2xx POST is a failure (#947)

CodeRabbit follow-up on #942:

- add_play_seconds() now stamps last_played_at (like touch_position):
  an unscored play that ran to the natural end WAS played — recent /
  Continue ordering must see it. Resume position stays untouched.
- stats-recorder post() treats non-2xx as failure: a 4xx/5xx JSON error
  body parsed as an object read as success, silently dropping the
  accrued seconds instead of re-queuing them.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

static/v3/stats-recorder.js

@@ -101,6 +101,10 @@
                 headers: { 'Content-Type': 'application/json' },
                 body: JSON.stringify(body),
             });
+            // A 4xx/5xx JSON error body must read as FAILURE — callers
+            // re-queue accrued seconds on null, and a parsed error object
+            // would silently drop them.
+            if (!r.ok) return null;
             try { return await r.json(); } catch (e) { return null; }
         } catch (e) { return null; /* offline / endpoint absent — non-fatal */ }
     }