docs(observability): refresh docs for service-registry end state

After the observability-service-registry slices removed the observability
env vars and the file-SD writer, several docs still instructed readers to
set vars that no longer exist. Updated the live config instructions;
historical decision-log entries are left intact.

- README.md: removed VITE_GRAFANA_URL/VITE_PROMETHEUS_URL/ALERTMANAGER_URL
  from compose examples and the env-var block; added a note that
  observability is configured on the Services page; updated Notes.
- frontend/README.md: dropped the stale VITE_* deep-link sentence.
- docs/REQUIREMENTS.md: fixed one stale trailing phrase in the
  externalization decision-log entry (VITE_* no longer "remain").
- docs/monitoring-logging-design.md: added a "Superseded mechanisms" note
  under Implementation Plan so the Phase 2/3 file-SD + alertmanager_url
  details read as historical, not current wiring.
- context.md: strengthened the status banner to cover the env->service-
  registry and file-SD->http_sd_configs shift; body marked historical.

.env.example is assistant-edit-blocked; updated replacement text provided
to the user separately.

README.md

@@ -88,17 +88,18 @@ export VITE_OIDC_ISSUER=https://auth.example.com/application/o/manage/
 export VITE_OIDC_CLIENT_ID=manage
 export VITE_OIDC_REDIRECT_URI=https://manage.example.com/oidc/callback
 export VITE_OIDC_POST_LOGOUT_REDIRECT_URI=https://manage.example.com/
-export VITE_GRAFANA_URL=https://grafana.manage.example.com
-export VITE_PROMETHEUS_URL=https://prometheus.manage.example.com
 export MANAGE_ENCRYPTION_KEY=$(python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())")
 
 docker compose up --build
 ```
 
+> Observability services (Grafana, Prometheus, Alertmanager) are configured in
+> the app on the **Services** page — no env vars for them.
+
 Inline one-liner example:
 
 ```bash
-BACKEND_APP_HOST=api.manage.example.com FRONTEND_APP_HOST=manage.example.com CERT_RESOLVER=letsencrypt VITE_OIDC_ISSUER=https://auth.example.com/application/o/manage/ VITE_OIDC_CLIENT_ID=manage VITE_OIDC_REDIRECT_URI=https://manage.example.com/oidc/callback VITE_OIDC_POST_LOGOUT_REDIRECT_URI=https://manage.example.com/ VITE_GRAFANA_URL=https://grafana.manage.example.com VITE_PROMETHEUS_URL=https://prometheus.manage.example.com docker compose up --build
+BACKEND_APP_HOST=api.manage.example.com FRONTEND_APP_HOST=manage.example.com CERT_RESOLVER=letsencrypt VITE_OIDC_ISSUER=https://auth.example.com/application/o/manage/ VITE_OIDC_CLIENT_ID=manage VITE_OIDC_REDIRECT_URI=https://manage.example.com/oidc/callback VITE_OIDC_POST_LOGOUT_REDIRECT_URI=https://manage.example.com/ docker compose up --build
 ```
 
 For local development, no SSH key is required unless you want to connect to remote SSH machines later:
@@ -145,9 +146,9 @@ VITE_OIDC_SCOPE=openid profile email
 VITE_OIDC_REDIRECT_URI=https://manage.example.com/oidc/callback
 VITE_OIDC_POST_LOGOUT_REDIRECT_URI=https://manage.example.com/
 
-# Grafana / Prometheus public URLs for frontend deep-links (service adapters read URLs from service records)
-VITE_GRAFANA_URL=https://grafana.manage.example.com
-VITE_PROMETHEUS_URL=https://prometheus.manage.example.com
+# Observability services (Grafana, Prometheus, Alertmanager) are configured in
+# the app on the Services page. The only observability env var is the optional
+# PROMETHEUS_ENABLED toggle (defaults on) for Manage's own /metrics endpoint.
 
 # Required: master key encrypting service secrets (API keys/tokens) at rest.
 # Generate one with: python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
@@ -185,5 +186,4 @@ cd frontend && npx tsc --noEmit && npm run build
 - Job templates are shell-quoted. Add new templates in `backend/src/media_library_viewer_api/jobs.py`.
 - Root-level Docker Compose files are provided for production (`docker-compose.yml`) and local development (`docker-compose.dev.yml`), and both rely on Compose interpolation rather than `env_file` entries. They deploy **only** the backend and frontend; Manage never deploys its own observability stack (see `docker-compose.observability.yml` for an optional standalone example).
 - The configurable dashboard stores widget instances in the backend SQLite settings database. New installs seed default Jellyfin activity and Backups widgets automatically.
-- Grafana and Prometheus widget adapters resolve URLs from service records configured in the app; `VITE_GRAFANA_URL` / `VITE_PROMETHEUS_URL` are only used for frontend deep-links. No credentials are stored in widget config; service API keys are encrypted at rest with `MANAGE_ENCRYPTION_KEY`.
-- `ALERTMANAGER_URL` is optional. When unset, the Alertmanager proxy endpoints return graceful "not configured" responses instead of erroring.
+- Grafana, Prometheus, and Alertmanager are configured as **service instances** in the app (Services page); their widget adapters resolve URLs from service records, and no observability URLs/credentials live in env vars. No credentials are stored in widget config; service API keys are encrypted at rest with `MANAGE_ENCRYPTION_KEY`. When no alertmanager service is configured, the alert proxy endpoints return graceful "not configured" responses.