feat(enterprise): add Advanced Reporting and Audit Webhooks integration

This commit adds enterprise-grade reporting and audit capabilities:

Reporting:
- Refactored metrics store from internal/ to pkg/ for enterprise access
- Added pkg/reporting with shared interfaces for report generation
- Created API endpoint: GET /api/admin/reports/generate
- New ReportingPanel.tsx for PDF/CSV report configuration

Audit Webhooks:
- Extended pkg/audit with webhook URL management interface
- Added API endpoint: GET/POST /api/admin/webhooks/audit
- New AuditWebhookPanel.tsx for webhook configuration
- Updated Settings.tsx with Reporting and Webhooks tabs

Server Hardening:
- Enterprise hooks now execute outside mutex with panic recovery
- Removed dbPath from metrics Stats API to prevent path disclosure
- Added storage metrics persistence to polling loop

Documentation:
- Updated README.md feature table
- Updated docs/API.md with new endpoints
- Updated docs/PULSE_PRO.md with feature descriptions
- Updated docs/WEBHOOKS.md with audit webhooks section

docs/API.md

@@ -144,6 +144,14 @@ Triggers a test alert to all configured channels.
 - `GET /api/notifications/email-providers` (admin)
 - `GET /api/notifications/health` (admin)
 
+### Audit Webhooks (Pro)
+- `GET /api/admin/webhooks/audit` (admin, `settings:read`)
+- `POST /api/admin/webhooks/audit` (admin, `settings:write`)
+
+### Advanced Reporting (Pro)
+- `GET /api/admin/reports/generate` (admin, `node:read`)
+  - Query params: `format` (pdf/csv), `id` (resource ID), `type` (node/vm/container/storage), `metric` (cpu/mem/avg), `range` (1h/24h/7d)
+
 ### Queue and Dead-Letter Tools
 - `GET /api/notifications/queue/stats` (admin)
 - `GET /api/notifications/dlq` (admin)