Remove End of Campaign from the Reports month dropdown

Already delivered as its own document, no need for it to stay
selectable in the live dashboard. The underlying generation code
(build_report_docx's campaign-end support) is left in place in case
that document needs regenerating later, just not exposed in the UI.

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

app.py

@@ -2177,7 +2177,6 @@ _REPORTS_PERIODS = [
     ("2026-07", "July 2026"),
     ("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]