feat(reports-p7): subtitle override field in dashboard builder

- DashboardReportBuilder gains an optional Subtitle input alongside
  Title. Persisted in the config payload sent to /api/v1/reports/runs
  + /api/v1/reports/generate + threaded through the preview payload's
  useMemo dep list so live preview reflects the override.
- Cover-page brand picker (admin-only) — deferred. Today the renderer
  uses the active port's brand kit; cross-port branding swap needs a
  permission gate, port-pick UI, and a renderer override and is queued
  for a follow-up. Subtitle alone covers the most common ad-hoc need
  (custom cover-page subtext like "Board pack — March 2026").

Verified: tsc clean, 1493/1493 vitest.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/reports/builders/dashboard-report-builder.tsx

@@ -50,6 +50,7 @@ interface Props {
 export function DashboardReportBuilder({ portSlug, initialFrom, initialTo }: Props) {
   const router = useRouter();
   const [title, setTitle] = useState(`Report - ${new Date().toLocaleDateString(undefined)}`);
+  const [subtitle, setSubtitle] = useState('');
   const [selected, setSelected] = useState<PdfDashboardWidgetId[]>(
     PDF_DASHBOARD_WIDGETS.map((w) => w.id),
   );
@@ -69,11 +70,12 @@ export function DashboardReportBuilder({ portSlug, initialFrom, initialTo }: Pro
       config: {
         kind: 'dashboard' as const,
         widgetIds: selected,
+        ...(subtitle.trim() ? { subtitle: subtitle.trim() } : {}),
         ...(dateFrom ? { dateFrom } : {}),
         ...(dateTo ? { dateTo } : {}),
       },
     }),
-    [title, selected, dateFrom, dateTo],
+    [title, subtitle, selected, dateFrom, dateTo],
   );
 
   function toggle(id: PdfDashboardWidgetId) {
@@ -102,6 +104,7 @@ export function DashboardReportBuilder({ portSlug, initialFrom, initialTo }: Pro
           config: {
             kind: 'dashboard',
             widgetIds: selected,
+            ...(subtitle.trim() ? { subtitle: subtitle.trim() } : {}),
             ...(dateFrom ? { dateFrom } : {}),
             ...(dateTo ? { dateTo } : {}),
           },
@@ -142,6 +145,8 @@ export function DashboardReportBuilder({ portSlug, initialFrom, initialTo }: Pro
           config: {
             kind: 'dashboard',
             widgetIds: selected,
+            title: title.trim() || 'Report',
+            ...(subtitle.trim() ? { subtitle: subtitle.trim() } : {}),
             ...(dateFrom ? { dateFrom } : {}),
             ...(dateTo ? { dateTo } : {}),
           },
@@ -193,6 +198,18 @@ export function DashboardReportBuilder({ portSlug, initialFrom, initialTo }: Pro
               className="max-w-md"
             />
           </div>
+          <div className="space-y-1">
+            <Label htmlFor="export-subtitle">
+              Subtitle <span className="text-xs font-normal text-muted-foreground">(optional)</span>
+            </Label>
+            <Input
+              id="export-subtitle"
+              value={subtitle}
+              onChange={(e) => setSubtitle(e.target.value)}
+              className="max-w-md"
+              placeholder="e.g. Board pack — March 2026"
+            />
+          </div>
           <div className="space-y-1">
             <Label>Report window</Label>
             <div className="flex flex-wrap items-center gap-2">