feat(api): add dashboard summary endpoint (#459)

Create GET /api/dashboard/summary aggregating 10 parallel Prisma
queries: task metrics, active agents, project counts, error rate,
recent activity, active runner jobs with steps, token budget entries.

Guarded by AuthGuard + WorkspaceGuard + PermissionGuard.
Includes DTO classes and unit tests.

Task: MS-P2-001

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

apps/api/src/dashboard/dto/index.ts

@@ -0,0 +1 @@
+export * from "./dashboard-summary.dto";