Add Uptime Kuma monitoring service

[xrhstosmour]

What:

1. Adds a self-hosted Uptime Kuma monitoring service at monitoring/services/uptime-kuma, following the existing docker-compose.yml + template.env pattern.
2. Keeps it self-contained on its own named volume, since upstream's stock image only documents SQLite, on the shared internal network with the port configurable via UPTIME_KUMA_PORT.

Why:

Pre-staging monitoring for a future VPS install.

Testing:

1. docker compose config validates with no errors.
2. docker compose up -d starts the container, reaches healthy status.
3. curl localhost:<port>/ returns a 302 redirect to the setup wizard.
4. docker compose down -v cleans up with no leftover state.