feat: MongoDB 8, Grafana dashboard JSON, Prometheus scrape config

- Upgraded from MongoDB 4.4 to 8 (AVX now available)
- Added grafana-dashboard.json with 4 rows: Overview stats, Host table, Volume/Duration trends, Time-since-last-backup
- Updated compose.yaml and README

README.md

@@ -206,10 +206,19 @@ The UI follows **The Sentinel** design language:
 ## Tech Stack
 
 - **Backend:** Python 3.12, Flask, Gunicorn
-- **Database:** MongoDB 4.4+
+- **Database:** MongoDB 8
 - **Frontend:** Tailwind CSS, Vanilla JS, Material Symbols, Google Fonts
 - **No build step** – Tailwind loaded via CDN
 
+## Grafana Dashboard
+
+Import `grafana-dashboard.json` into Grafana for a pre-built dashboard with:
+- Overview stat panels (Hosts OK, Stale, Errors, Volume, Backups Today)
+- Host status table with color-coded status, age, duration, size
+- Backup volume per host (stacked time series)
+- Backup duration per host
+- Time since last backup with threshold coloring (green/yellow/red)
+
 ## License
 
 MIT