Add October 2026 to the Reports month dropdown

The 3-month campaign (July-September) wrapped, but posts have kept
going (01/10 already up). October wasn't selectable in the Reports
tab at all - added it to _REPORTS_PERIODS. Falls back to "Executive
Summary not written up yet" until October's narrative is written,
same graceful handling already in place for any unwritten month.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app.py

@@ -2178,6 +2178,7 @@ _REPORTS_PERIODS = [
     ("2026-08", "August 2026"),
     ("2026-09", "September 2026"),
     ("campaign-end", "End of Campaign"),
+    ("2026-10", "October 2026"),
 ]
 _REPORTS_MONTH_OPTIONS = [{"label": label, "value": key} for key, label in _REPORTS_PERIODS]
 _REPORTS_DEFAULT_MONTH = "2026-07"