fix(python): return months_to_show from the JSON API

Go returns months_to_show in /api/adults and /api/juniors; the Python
API did not, so make parity failed on both routes. The Python HTML
pages already had the feature.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

legacy/app.py

@@ -179,7 +179,9 @@ def api_adults():
         datetime.now().strftime("%Y-%m"),
         attendance_url=attendance_url, payments_url=payments_url, bank_account=BANK_ACCOUNT,
     )
-    return jsonify(_unwrap_view_model_for_api(vm))
+    out = _unwrap_view_model_for_api(vm)
+    out["months_to_show"] = MONTHS_TO_SHOW
+    return jsonify(out)
 
 @app.route("/api/juniors")
 def api_juniors():
@@ -203,7 +205,9 @@ def api_juniors():
         datetime.now().strftime("%Y-%m"),
         attendance_url=attendance_url, payments_url=payments_url, bank_account=BANK_ACCOUNT,
     )
-    return jsonify(_unwrap_view_model_for_api(vm))
+    out = _unwrap_view_model_for_api(vm)
+    out["months_to_show"] = MONTHS_TO_SHOW
+    return jsonify(out)
 
 @app.route("/api/payments")
 def api_payments():