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- Remove addon pages (/addons/:addonId, AddonPage, addons/*) superseded by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.

.env.example

@@ -27,8 +27,6 @@ PROMETHEUS_ENABLED=true
 PROMETHEUS_FILE_SD_DIR=/app/backend/.cache/prometheus-file-sd
 ALERTMANAGER_URL=http://alertmanager:9093
 ALERTMANAGER_WEBHOOK_URL=
-GRAFANA_URL=http://grafana:3000
-PROMETHEUS_URL=http://prometheus:9090
 # Required: master key for encrypting service secrets (API keys/tokens) at rest.
 # Generate one with: python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
 MANAGE_ENCRYPTION_KEY=replace-with-a-fernet-key