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.

docker-compose.yml

@@ -34,8 +34,6 @@ services:
       SMTP_USE_SSL: ${SMTP_USE_SSL:-false}
       SMTP_TIMEOUT: ${SMTP_TIMEOUT:-30}
       SSH_KNOWN_HOSTS_PATH: /app/backend/.cache/known_hosts
-      ALERTMANAGER_URL: ${ALERTMANAGER_URL:-}
-      ALERTMANAGER_WEBHOOK_URL: ${ALERTMANAGER_WEBHOOK_URL:-}
       MANAGE_ENCRYPTION_KEY: ${MANAGE_ENCRYPTION_KEY:?generate one with python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"}
     volumes:
       - ${BACKEND_CACHE_DIR:-./backend-cache}:/app/backend/.cache
@@ -77,8 +75,6 @@ services:
         VITE_OIDC_REDIRECT_URI: ${VITE_OIDC_REDIRECT_URI:?set VITE_OIDC_REDIRECT_URI}
         VITE_OIDC_POST_LOGOUT_REDIRECT_URI: ${VITE_OIDC_POST_LOGOUT_REDIRECT_URI:?set VITE_OIDC_POST_LOGOUT_REDIRECT_URI}
         VITE_DEV_API_PROXY_TARGET: ${VITE_DEV_API_PROXY_TARGET:-http://backend:8000}
-        VITE_GRAFANA_URL: ${VITE_GRAFANA_URL:-https://grafana.example.com}
-        VITE_PROMETHEUS_URL: ${VITE_PROMETHEUS_URL:-http://prometheus.example.com}
         VITE_APP_VERSION: ${APP_VERSION:-0.1.0}
         VITE_APP_BUILD_INFO: ${APP_BUILD_INFO:-dev}
     depends_on: