chore(observability): remove remaining observability env vars, docs

Slice 5 (final) of observability-service-registry. Completes the move to
service-registry-only observability config: no observability service env
vars remain.

- config.py: removed alertmanager_url + alertmanager_webhook_url fields.
- docker-compose.yml / docker-compose.dev.yml: removed ALERTMANAGER_URL,
  ALERTMANAGER_WEBHOOK_URL (backend env), and VITE_GRAFANA_URL,
  VITE_PROMETHEUS_URL (frontend build args / dev env).
- frontend/Dockerfile: removed the VITE_GRAFANA_URL / VITE_PROMETHEUS_URL
  ARG, build-stage ENV, and dev-stage ENV lines.
- docs: REQUIREMENTS decision-log entry; CHANGELOG Added/Changed/BREAKING
  for the observability service registry; backend/README monitoring
  section (Observability page, services page config, http_sd_configs,
  new health endpoints, log-only webhook).

The only observability env var remaining is PROMETHEUS_ENABLED (Manage's
own /metrics toggle). Grep-gated: no live references to the removed
vars/fields in backend src, frontend src, compose, or Dockerfile.

ruff clean; 239 backend tests pass; frontend 0 lint errors, build clean,
72 tests pass.

.env.example is assistant-edit-blocked; user follow-up noted in the SDD
tasks: drop the removed vars there too.

backend/src/media_library_viewer_api/config.py

@@ -54,8 +54,6 @@ class Settings(BaseSettings):
 
     # Observability
     prometheus_enabled: bool = True
-    alertmanager_url: str = ""
-    alertmanager_webhook_url: str = ""  # Optional receiver for alertmanager webhook notifications
 
     # Remote paths
     remote_media_root: str = ""