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.

CHANGELOG.md

@@ -4,6 +4,20 @@ All notable changes to Manage. Breaking changes are marked with **BREAKING**.
 
 ## [Unreleased]
 
+### Added — Observability service registry
+
+- **Alertmanager is now a service type.** Configure Alertmanager, Grafana, and
+  Prometheus instances in the UI on the Services page; all three are first-class
+  service-registry entries with dashboard widgets (`active_alerts`, Grafana link,
+  Prometheus metric).
+- New monitoring endpoints resolve the configured service instance and probe its
+  health: `GET /api/monitoring/grafana-status`, `/prometheus-status`. The
+  `/alerts` and `/alertmanager-status` endpoints now take an optional
+  `service_id` and pick the first enabled alertmanager instance by default.
+- The Observability page discovers Grafana/Prometheus/Alertmanager from the
+  registry and renders health cards; the dashboard `active_alerts` widget sums
+  firing alerts by severity.
+
 ### Changed — Observability is now external only
 
 - **Removed** all observability services from `docker-compose.yml` and
@@ -15,13 +29,22 @@ All notable changes to Manage. Breaking changes are marked with **BREAKING**.
   and never ships its own stack. The previous in-compose stack is preserved as
   an optional, deploy-it-yourself example in `docker-compose.observability.yml`
   (config under `monitoring/`, documented in `docs/observability-runbooks.md`).
-- The backend `alertmanager_url` default is now empty. The
-  `/api/monitoring/alerts` and `/alertmanager-status` endpoints return a
-  graceful "not configured" response when `ALERTMANAGER_URL` is unset.
 - Removed the now-orphaned combined `monitoring/prometheus/prometheus.yml`; the
   standalone stack uses `monitoring/prometheus/prometheus.standalone.yml`.
-- `VITE_GRAFANA_URL`/`VITE_PROMETHEUS_URL` remain as optional frontend deep-link
-  overrides. `ALERTMANAGER_URL`/`ALERTMANAGER_WEBHOOK_URL` are optional.
+- Removed the Prometheus file-SD bridge (`PROMETHEUS_FILE_SD_DIR` + the
+  `write_prometheus_targets` file writer). External Prometheus instances now
+  consume node-exporter targets via `http_sd_configs` against
+  `GET /api/monitoring/prometheus-targets`. The webhook receiver is log-only.
+
+### **BREAKING**
+
+- Observability is configured entirely via the service registry; the backend
+  `alertmanager_url`/`alertmanager_webhook_url` and frontend
+  `VITE_GRAFANA_URL`/`VITE_PROMETHEUS_URL` environment variables, plus
+  `PROMETHEUS_FILE_SD_DIR`, were **removed**. Re-create your Alertmanager /
+  Grafana / Prometheus instances on the Services page after upgrading. The only
+  observability env var remaining is `PROMETHEUS_ENABLED` (toggles Manage's own
+  `/metrics` endpoint).
 
 ### Added — Service registry