feat(widgets): dashboard loop, widget config UI, and addon pages

PR 4 of 4 for configurable dashboard widgets.

- Replace hard-coded Jellyfin/Backups dashboard sections with a loop that
  renders enabled widget instances by sort_order.
- Add WidgetInstance renderer and WidgetConfigDialog for adding, editing,
  enabling/disabling, deleting, and reordering widgets.
- Add addon pages for grafana, prometheus, and ssh-tasks at /addons/:addonId.
- Register /addons/:addonId route in App.tsx.
- Update docs/REQUIREMENTS.md with the widget system design and API.

Verification:
- backend ruff clean; pytest 200 passed
- frontend npm run lint: 0 errors
- frontend npm run build: success
- frontend npm run test -- src/widgets/registry.test.ts: 3 passed

docs/REQUIREMENTS.md

@@ -256,6 +256,54 @@ fully removed (web-ui-rework; see decision log 2026-06-17).
 - Job templates should remain centralized in `jobs.py` for future extension.
 - Remote job template values must be shell-quoted before execution.
 
+## Configurable Dashboard Widgets
+
+### Overview
+
+The dashboard is composed of persisted widget instances stored in the backend SQLite
+settings database. Each widget has a type, title, configuration, enabled flag, and
+sort order. The frontend renders enabled widgets in sort order and fetches data
+independently through the backend source adapters.
+
+### Widget types
+
+- **Jellyfin activity** — live sessions and idle users from a configured Jellyfin machine.
+- **Backups** — backup job summary and active alerts.
+- **Grafana link** — deep-link to a Grafana dashboard or panel (no iframe embedding).
+- **Prometheus metric** — result of a PromQL instant query.
+- **SSH task output** — output of a saved task run on a machine.
+- **Static text** — plain text or markdown note.
+
+### Security
+
+- Widget `config` may not contain credential keys such as `password`, `token`,
+  `secret`, `api_key`, `private_key`, or `passphrase`, or values that look like
+  secrets (e.g., base64 blobs, `sk-` prefixes).
+- Widgets reuse machine-level Jellyfin/SSH credentials and environment settings for
+  Grafana/Prometheus URLs; no secrets are stored in widget configuration.
+- SSH task widgets only run tasks from the saved-task registry; arbitrary commands
+  are not accepted.
+
+### Addon pages
+
+Each non-core addon gets a dedicated page at `/addons/:addonId`:
+
+- `/addons/grafana`
+- `/addons/prometheus`
+- `/addons/ssh-tasks`
+
+Unknown addons render a "not installed" alert.
+
+### API
+
+- `GET /api/widgets/sources` — list source types.
+- `GET /api/widgets/types` — list widget type metadata.
+- `GET /api/widgets/instances` — list widget instances.
+- `POST /api/widgets/instances` — create instance.
+- `PUT /api/widgets/instances/{id}` — update instance.
+- `DELETE /api/widgets/instances/{id}` — delete instance.
+- `GET /api/widgets/instances/{id}/data` — fetch widget data.
+
 ## Decision Log
 
 - 2026-06-17: Decommissioned the legacy Manage-side system-metric scraping. Removed the backend `MonitoringPoller` (SSH-ran `df` on every machine every 5 min into a local SQLite `monitoring_machine_actions` table), the entire `services/monitoring_actions.py` module, the `/api/monitoring/poller`, `/api/monitoring/machines/{id}/actions`, and `/api/monitoring/disk` endpoints, the `monitoring_machine_actions` table (DROP on startup), the three `monitoring_poll_*` / `monitoring_action_retention_days` config knobs, and the orphaned frontend `DiskSpaceCard` + `DiskSpace` type. System metrics are now owned exclusively by Prometheus + node_exporter + Grafana. Kept the Alertmanager proxy (`/alerts`, `/alertmanager-status`, `/alertmanager-webhook`), `/prometheus-targets`, `/machines`, the `node_exporter_*` machine fields, and the on-demand `disk_usage` job template.