chore(observability): externalize stack from root compose files

Manage now connects to existing Grafana/Prometheus/Alertmanager instances
and never deploys its own stack.

- docker-compose.yml / docker-compose.dev.yml: removed prometheus, loki,
  alloy, grafana, alertmanager, node-exporter services, the monitoring
  network, and observability named volumes; they now ship only backend +
  frontend. Dev frontend now joins the web network so the Vite dev proxy
  can reach the backend.
- backend: alertmanager_url default is now empty; /api/monitoring/alerts
  and /alertmanager-status return graceful "not configured" responses
  when ALERTMANAGER_URL is unset. Added not-configured tests.
- docker-compose.observability.yml: kept as the optional standalone
  example; header clarifies Manage does not deploy it.
- Removed orphaned combined monitoring/prometheus/prometheus.yml
  (standalone stack uses prometheus.standalone.yml).
- Docs (README, REQUIREMENTS decision log, monitoring-logging-design,
  observability-runbooks, context.md, MIGRATION_PLAN, frontend/README,
  CHANGELOG) updated to the connect-to-existing model.

VITE_GRAFANA_URL / VITE_PROMETHEUS_URL remain as optional frontend
deep-link overrides. .env.example still needs a manual update (safety
policy blocks assistant edits): set ALERTMANAGER_URL empty/optional and
move standalone-only vars out of the root file.

frontend/README.md

@@ -8,10 +8,9 @@ React + TypeScript SPA for Manage, consuming the FastAPI backend.
 - **React 18+** — UI framework
 - **TypeScript** — Type safety
 - **@tanstack/react-query** — Data fetching/caching
-- **ag-grid-react** — Data tables (media, file browser)
-- **recharts** — Monitoring charts
+- **@tanstack/react-table** — Data tables (media, file browser)
 - **react-router-dom** — Client-side routing
-- **Tailwind CSS** — Styling
+- **Tailwind CSS** + **shadcn/ui** — Styling
 
 ## Setup
 
@@ -26,7 +25,7 @@ npm install
 npm run dev
 ```
 
-Runs on http://localhost:5173 with API requests proxied to http://localhost:8000.
+Runs on <http://localhost:5173> with API requests proxied to <http://localhost:8000>.
 
 Make sure the backend is running:
 
@@ -45,12 +44,13 @@ Output goes to `frontend/dist/`.
 
 ## Pages
 
-- **Dashboard** (`/`) — Now playing, backend-collected per-machine monitoring table with 10-minute averages/min/max, library stats, frontend/backend version chips in the shell header
-- **Monitoring** (`/monitoring`) — Per-machine CPU/IO wait/RAM/network/disk charts, collector controls, and backend-collected recent action history
+- **Dashboard** (`/`) — Now playing, library stats, configurable widgets and shortcuts, frontend/backend version chips in the shell header
+- **Observability** (`/observability`) — Thin dashboard: Alertmanager alerts, Prometheus target health, machine status, and Grafana deep-links (no in-app charting)
 - **Media** (`/media`) — Full-library table with sort/filter/search
 - **Users** (`/users`) — Read-only Jellyfin user list with optional Jellyseerr enrichment
 - **File Browser** (`/files`) — Remote directory browsing, ffprobe preview, jobs
-- **Settings** (`/settings`) — Persistent monitoring machine definitions and setup workflow
+- **Settings** (`/settings`) — Persistent monitoring machine definitions, SSH keys, and setup workflow
+- **Actions** (`/actions`) — Saved server tasks (shell/python) targeting ssh_tasks services
 
 ## Environment Variables
 
@@ -65,7 +65,6 @@ VITE_API_URL=http://your-backend-host:8000
 
 - **Local development**: run `docker compose -f docker-compose.dev.yml up --build`, then open the app and add monitoring machines in the **Settings** tab.
 - **Production**: export the required Compose variables in your shell, run `docker compose up --build`, and manage local/remote machines from **Settings**.
-- **Dashboard monitoring UI**: the dashboard shows a compact, sortable table with one row per configured machine plus poller status.
-- **Monitoring UI**: the **Monitoring** tab shows one card per configured machine, including a recent action-history table populated by the backend poller. A machine can be `local` (the API host itself) or `ssh` (a remote host), and the UI treats both the same after configuration. The page also shows the backend poller health badge.
+- **Observability**: Manage only deploys backend + frontend. It connects to **existing** Grafana/Prometheus/Alertmanager instances; see `docker-compose.observability.yml` for an optional standalone example stack. A machine can be `local` (the API host itself) or `ssh` (a remote host), and the UI treats both the same after configuration.
 
-In development, the Vite proxy handles `/api` requests automatically.
+In development, the Vite proxy handles `/api` requests automatically. `VITE_GRAFANA_URL` / `VITE_PROMETHEUS_URL` are optional frontend deep-link overrides for your existing instances.