Add document storage (local filesystem) + fix dashboard outstanding KPI

- Local file storage via STORAGE_PATH env var (replaces DO Spaces)
- POST/GET/DELETE /api/cases/:caseId/documents + GET /api/documents/:docId/download
- useDocuments hook + upload/list/download/delete UI in CaseDetailPage
- GET /api/invoices/summary — live SUM of sent+overdue invoices
- Dashboard outstanding KPI wired to real DB value
- api.upload() for FormData, formatBytes() utility

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/lib/format.ts

@@ -16,6 +16,12 @@ export function formatMoney(amount: string | number | null | undefined): string
   return new Intl.NumberFormat(undefined, { style: 'currency', currency: 'USD' }).format(n);
 }
 
+export function formatBytes(bytes: number): string {
+  if (bytes < 1024) return `${bytes} B`;
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+}
+
 export function planLimitMessage(code: string | undefined, fallback = 'Action not allowed.'): string {
   switch (code) {
     case 'plan_limit_clients':