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>

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## 2026-05-08 11:11 CEST — fix: period selector showed only Dec 2025+ on adults
+
+- Restored the `2025-09 → 2025-10` adult merge in `scripts/attendance.py` and `go/internal/services/membership/sources.go` (commented-out by `1257f0d`); the `2025-12 → 2026-01` mapping stays disabled per product decision (Dec and Jan are billed separately for adults).
+- Dropped the `defaultFrom = maxMonthIdx − 4` JS auto-default in `templates/adults.html` and `templates/juniors.html` (introduced by `7774301`); the From-selector now starts at the oldest available month so all non-future periods render on first load. Future-month removal is preserved.
+- `go/internal/services/membership/sources_test.go`: `TestLoadAdults` / `TestLoadAdults_Fee` now assert that Sep dates land in the merged `2025-10` bucket.
+- **Independent of these code changes**: the live adults attendance Google Sheet header had been pruned to start at `02.12.2025` (Sep/Oct/Nov 2025 columns deleted); restoring those columns from Sheets version history is required to actually see those periods on the dashboard.
+
 ## 2026-05-08 10:15 CEST — fix(go): adults template — use lifted CSS classes for visual parity
 
 - Use existing `.balance-pos` / `.balance-neg` (drop invented `balance-cell` / `balance-negative`); Pay-All button now lives inside the balance cell with `position: relative` (matches Python; no separate trailing column).