Pin alertmanager image version (currently uses :latest)

[mvillmow]

The alertmanager service uses prom/alertmanager:latest while every other service in docker-compose.yml uses a pinned version (e.g., prom/prometheus:v2.54.1, grafana/grafana:11.2.2). This is an inconsistency that can cause silent breakage if Alertmanager ships a breaking config change. Pin to a specific release such as v0.27.0 to match the project's version-pinning policy.

Follow-up from #107

[mvillmow]

Verified ALREADY-DONE. Evidence documented in PR #502 body: prom/alertmanager:v0.32.1 is already pinned in docker-compose.yml. Closing as no further work needed.