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/api/src/env.ts

@@ -19,6 +19,7 @@ const envSchema = z.object({
   WEB_DIST_PATH: z.string().optional(),
   SUPERADMIN_EMAILS: z.string().optional().default(''),
   SENTRY_DSN_API: z.string().optional().default(''),
+  STORAGE_PATH: z.string().min(1).default('./storage'),
   RESEND_API_KEY: z.string().optional().default(''),
   EMAIL_FROM: z.string().optional().default('eLegal Software <noreply@elegalsoftware.com>'),
   STRIPE_SECRET_KEY: z.string().optional().default(''),