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- Remove addon pages (/addons/:addonId, AddonPage, addons/*) superseded by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

backend/src/media_library_viewer_api/config.py

@@ -57,8 +57,6 @@ class Settings(BaseSettings):
     prometheus_file_sd_dir: str = "/app/backend/.cache/prometheus-file-sd"
     alertmanager_url: str = "http://alertmanager:9093"
     alertmanager_webhook_url: str = ""  # Optional receiver for alertmanager webhook notifications
-    grafana_url: str = "http://grafana:3000"
-    prometheus_url: str = "http://prometheus:9090"
 
     # Remote paths
     remote_media_root: str = ""