fix(display): default from-selector to last N months; keep all months selectable

Instead of hiding older months entirely, show all months in the from/to
selectors but default the from-select to the last MONTHS_TO_SHOW months
on page load. The "All" button resets to full history as before.

Python: passes months_to_show to render_template, IIFE sets fromSelect.value.
Go: adds MonthsToShow to response structs, data-months-to-show attr in
templates, filters.js reads it and defaults fromSelect after hideFutureMonths.

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

go/tests/fixtures/api-schema/adults.schema.json

@@ -143,6 +143,9 @@
         },
         "current_month": {
           "type": "string"
+        },
+        "months_to_show": {
+          "type": "integer"
         }
       },
       "additionalProperties": false,
@@ -161,7 +164,8 @@
         "attendance_url",
         "payments_url",
         "bank_account",
-        "current_month"
+        "current_month",
+        "months_to_show"
       ]
     },
     "Credit": {