docs(openspec): add SDD artifacts for next changes

- jellyfin-service-registry: proposal, design, and tasks for completing
  the Jellyfin migration off machine-level config.
- grafana-prometheus-polish: proposal, design, and tasks for improving
  the Grafana/Prometheus observability integration.

Both are planning-only artifacts; implementation not started.

openspec/changes/jellyfin-service-registry/design.md

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+# Design — Jellyfin service-registry migration (completion)
+
+**Change:** `jellyfin-service-registry`
+**Phase:** design
+**Date:** 2026-06-23
+
+## Current state
+
+- Jellyfin/Jellyseerr service definitions exist in `integrations/jellyfin.py` and `integrations/jellyseerr.py`.
+- `dependencies.py` resolves Jellyfin clients from service records via `jellyfin_service_id`.
+- Frontend pages already select Jellyfin services via `useServiceInstances("jellyfin")`.
+- `settings_store.py` still defaults machine `services` to `["monitoring", "files", "jellyfin"]` and persists `media_root` / `path_prefix`.
+- `MonitoringMachine` / `MonitoringMachineInput` types still include `media_root` and `path_prefix`.
+- `Settings.tsx` still renders `media_root` and `path_prefix` inputs and lists `jellyfin` as a service option.
+- `path_utils.py` still has path-mapping helpers, but the callers in `files.py` no longer depend on machine-level `media_root`/`path_prefix`.
+
+## Target state
+
+### Backend
+
+- `DEFAULT_SERVICES` becomes `["monitoring", "files"]`.
+- `settings_store.py` drops `media_root` and `path_prefix` from `_normalize_machine_payload`, `_row_to_machine`, the machine table schema, and default seeding.
+- Add `DROP TABLE IF EXISTS ...` or `ALTER TABLE DROP COLUMN` cleanup as needed for existing databases (SQLite has limited `DROP COLUMN` support; prefer table rebuild or leave harmless columns).
+- `path_utils.py` is reviewed; if no callers remain, delete the module. If callers remain, ensure they do not reference machine fields.
+- `routers/files.py` confirmed to not read machine `media_root`/`path_prefix`.
+- Tests updated to remove `media_root`/`path_prefix` from fixtures.
+
+### Frontend
+
+- Remove `media_root` and `path_prefix` from `MonitoringMachine` and `MonitoringMachineInput` in `types/index.ts`.
+- Remove `jellyfin` from the machine services multi-select in `Settings.tsx`.
+- Remove `media_root` and `path_prefix` inputs from the machine editor.
+- Update `emptyMachine()` to not include these fields.
+- Update tests/fixtures.
+
+### Docs
+
+- `docs/REQUIREMENTS.md` — update the machine config description to remove Jellyfin/path-prefix fields.
+- `CHANGELOG.md` — breaking note about removed machine fields.
+
+## Slices
+
+1. **Backend cleanup** — `settings_store.py`, `config.py` (if any env vars), `path_utils.py`, tests.
+2. **Frontend cleanup** — types, `Settings.tsx`, tests.
+3. **Docs + changelog**.
+
+## Risks
+
+- Existing DB rows may have `media_root`/`path_prefix` values; dropping columns is safe but irreversible. Backups recommended.
+- `path_utils.py` may still be referenced by archive code; verify only live code is affected.