feat(display): limit /adults and /juniors to last N months by default

Show only the last MONTHS_TO_SHOW months (default 5) in the fee table columns
so the page fits on screen without horizontal scrolling. Reconciliation still
runs over the full month history so balances, credits, and debts are unaffected.
Set MONTHS_TO_SHOW=0 to show all months. Implemented in both Python and Go.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/config.py

@@ -40,6 +40,9 @@ DRIVE_TIMEOUT = 10  # seconds
 CACHE_TTL_SECONDS = int(os.environ.get("CACHE_TTL_SECONDS", 300))  # 5 min default
 CACHE_API_CHECK_TTL_SECONDS = int(os.environ.get("CACHE_API_CHECK_TTL_SECONDS", 300))  # 5 min default
 
+# Display settings
+MONTHS_TO_SHOW = int(os.environ.get("MONTHS_TO_SHOW", 5))  # show last N months; 0 = show all
+
 # Maps cache keys to their source sheet IDs (used by cache_utils)
 CACHE_SHEET_MAP = {
     "attendance_regular": ATTENDANCE_SHEET_ID,