fix: restore Sep+Oct adult merge and stop auto-truncating period selector

Two regressions made older periods invisible on the adults dashboard:

- 1257f0d (Mar 9) commented out ADULT_MERGED_MONTHS, removing the
  Sep+Oct 2025 merged label. Restored only the 2025-09 → 2025-10
  mapping (Dec and Jan are billed separately for adults; the
  Dec → Jan mapping stays disabled per product decision). Mirrored
  on the Go side. Test fixtures in sources_test.go now assert Sep
  dates land in merged 2025-10 instead of 2025-09.

- 7774301 (Apr 9) added a JS onload default that set the From
  selector to maxMonthIdx − 4 and immediately filtered the table,
  hiding everything older than 5 months on first load. Dropped that
  default in templates/adults.html and templates/juniors.html so
  the From-selector starts at the oldest available month. Future
  months are still removed from the dropdowns and hidden in the
  table — only the past-month truncation is gone.

Note: the live adults attendance sheet had also been pruned to
start at 02.12.2025; restoring Sep/Oct/Nov 2025 columns from
Sheets version history is required to actually see those periods.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/services/membership/sources.go

@@ -27,7 +27,9 @@ const (
 
 // AdultMergedMonths mirrors ADULT_MERGED_MONTHS in scripts/attendance.py.
 // Source month → target month (source attendance accumulated into target).
-var AdultMergedMonths = map[string]string{}
+var AdultMergedMonths = map[string]string{
+	"2025-09": "2025-10",
+}
 
 // JuniorMergedMonths mirrors JUNIOR_MERGED_MONTHS in scripts/attendance.py.
 var JuniorMergedMonths = map[string]string{