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.

frontend/Dockerfile

@@ -15,8 +15,6 @@ ARG VITE_OIDC_SCOPE=openid profile email
 ARG VITE_OIDC_REDIRECT_URI=
 ARG VITE_OIDC_POST_LOGOUT_REDIRECT_URI=
 ARG VITE_DEV_API_PROXY_TARGET=http://backend:8000
-ARG VITE_GRAFANA_URL=https://grafana.example.com
-ARG VITE_PROMETHEUS_URL=http://localhost:9090
 ARG VITE_APP_VERSION=0.1.0
 ARG VITE_APP_BUILD_INFO=dev
 
@@ -28,8 +26,6 @@ ENV VITE_API_URL=${VITE_API_URL} \
     VITE_OIDC_REDIRECT_URI=${VITE_OIDC_REDIRECT_URI} \
     VITE_OIDC_POST_LOGOUT_REDIRECT_URI=${VITE_OIDC_POST_LOGOUT_REDIRECT_URI} \
     VITE_DEV_API_PROXY_TARGET=${VITE_DEV_API_PROXY_TARGET} \
-    VITE_GRAFANA_URL=${VITE_GRAFANA_URL} \
-    VITE_PROMETHEUS_URL=${VITE_PROMETHEUS_URL} \
     VITE_APP_VERSION=${VITE_APP_VERSION} \
     VITE_APP_BUILD_INFO=${VITE_APP_BUILD_INFO}
 
@@ -54,8 +50,6 @@ COPY frontend/ ./
 ENV VITE_API_URL=/api \
     VITE_OIDC_ENABLED=false \
     VITE_DEV_API_PROXY_TARGET=http://backend:8000 \
-    VITE_GRAFANA_URL=http://localhost:3000 \
-    VITE_PROMETHEUS_URL=http://localhost:9090 \
     VITE_APP_VERSION=0.1.0 \
     VITE_APP_BUILD_INFO=dev