feat(services): cleanup, services admin UI, docs

PR 4a of the runtime service registry change.

- Remove addon pages (/addons/:addonId, AddonPage, addons/*) superseded by
  service pages.
- Remove grafana_url/prometheus_url from backend config, compose, .env.example,
  and README (URLs now live on service records; VITE_ frontend deep-link vars
  retained).
- Add Services page (/services) with create/list/delete + sidebar nav, so
  services are configurable in the tool itself and service pages are reachable.
- Update docs/REQUIREMENTS.md service-registry section; add CHANGELOG.md with
  the breaking-upgrade note (MANAGE_ENCRYPTION_KEY required; grafana/prometheus
  env vars removed; default widget seeding removed).

Verification: backend ruff clean, pytest 222 passed; frontend lint 0 errors,
build success, 70 tests passed.

docker-compose.dev.yml

@@ -17,8 +17,6 @@ services:
       PROMETHEUS_FILE_SD_DIR: /app/backend/.cache/prometheus-file-sd
       ALERTMANAGER_URL: ${ALERTMANAGER_URL:-http://alertmanager:9093}
       ALERTMANAGER_WEBHOOK_URL: ${ALERTMANAGER_WEBHOOK_URL:-}
-      GRAFANA_URL: ${GRAFANA_URL:-http://grafana:3000}
-      PROMETHEUS_URL: ${PROMETHEUS_URL:-http://prometheus:9090}
       MANAGE_ENCRYPTION_KEY: ${MANAGE_ENCRYPTION_KEY:?set MANAGE_ENCRYPTION_KEY in your .env}
     ports:
       - "8000:8000"