feat(reports): Excel renderer + attachment + archive download

render_excel(report): one worksheet per location — interval table, a line chart,
and a Last/Base/Δ summary per window. Metric-driven, so it tracks whatever metric
set is configured.

- orchestrator: render report.xlsx alongside report.html, attach it to the email
  (dry-run until SMTP set), expose xlsx_path. Never lets a spreadsheet error sink
  the report.
- reports router: /list includes xlsx_url when present; new
  GET /archive/{date}/xlsx serves the saved spreadsheet.
- UI: Recent-reports rows get an "Excel" download link.

Verified: real Feb data -> valid .xlsx (sheet per NRL, interval table + chart +
summary with real values), attachment path runs, both archive routes registered.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

templates/partials/projects/project_header.html

@@ -206,9 +206,10 @@ function loadRecentReports(projectId) {
             }
             box.innerHTML = j.reports.map(function (rp) {
                 var when = (rp.generated_at || '').replace('T', ' ').slice(0, 16);
-                return '<a href="' + rp.view_url + '" target="_blank" class="flex items-center justify-between px-3 py-2 text-sm hover:bg-gray-50 dark:hover:bg-gray-700">'
-                     + '<span class="font-medium text-gray-800 dark:text-gray-200">Night of ' + rp.night_date + '</span>'
-                     + '<span class="text-xs text-gray-400">' + when + ' UTC</span></a>';
+                var xlsx = rp.xlsx_url ? ' · <a href="' + rp.xlsx_url + '" class="text-indigo-600 dark:text-indigo-400 hover:underline">Excel</a>' : '';
+                return '<div class="flex items-center justify-between px-3 py-2 text-sm">'
+                     + '<a href="' + rp.view_url + '" target="_blank" class="font-medium text-gray-800 dark:text-gray-200 hover:underline">Night of ' + rp.night_date + '</a>'
+                     + '<span class="text-xs text-gray-400">' + when + ' UTC' + xlsx + '</span></div>';
             }).join('');
         })
         .catch(function () { box.innerHTML = '<div class="px-3 py-2 text-xs text-red-500">Failed to load.</div>'; });