docs: complete Jellyfin migration, archive jellyfin-service-registry

Slice 3 (final) of jellyfin-service-registry. Documents the completed
migration and archives the SDD change.

- docs/REQUIREMENTS.md: marked the machine-level Jellyfin follow-up
  resolved; added a decision-log entry (Jellyfin no longer a machine
  service, dead media_root/path_prefix removed; global config +
  path_utils retained for Jellyfin->SSH path resolution).
- CHANGELOG.md: struck through the old follow-up note; added a
  Follow-up #2 section describing the machine field + service removal.
- Archived openspec/changes/jellyfin-service-registry (no active SDD
  changes remain).

Backend ruff clean / 239 tests pass; frontend 0 lint errors / build
clean / 72 tests pass.

openspec/changes/archive/jellyfin-service-registry/tasks.md

@@ -0,0 +1,86 @@
+# Tasks — Jellyfin service-registry migration (completion)
+
+**Change:** `jellyfin-service-registry`
+**Phase:** tasks
+**Date:** 2026-06-23
+
+## Review workload forecast
+
+| Field | Value |
+|-------|-------|
+| Estimated changed lines | ~250–350 |
+| Chained PRs recommended | No (single PR) |
+| Chain strategy | n/a |
+
+## Slice 1: Backend cleanup
+
+**Goal:** Remove Jellyfin and dead path-mapping fields from machine config.
+
+- [ ] **1.1 Drop `jellyfin` from default machine services**
+  - Files: `backend/src/media_library_viewer_api/services/settings_store.py`
+  - Lines: ~5
+  - Details: Change `DEFAULT_SERVICES = ["monitoring", "files", "jellyfin"]` to `["monitoring", "files"]`. Update all fallback/default service lists.
+
+- [ ] **1.2 Remove `media_root` and `path_prefix` machine fields**
+  - Files: `backend/src/media_library_viewer_api/services/settings_store.py`
+  - Lines: ~40
+  - Details: Remove from machine table schema (if present), `_normalize_machine_payload`, `_row_to_machine`, default seeding, and any serialization helpers.
+
+- [ ] **1.3 Review/delete `path_utils.py`**
+  - Files: `backend/src/media_library_viewer_api/path_utils.py`, `backend/tests/test_path_utils.py`
+  - Lines: ~-120 if deleted, else ~10 to confirm callers
+  - Details: Check if any live router still imports the helpers. If not, delete module and tests.
+
+- [ ] **1.4 Update backend tests**
+  - Files: `backend/tests/test_api.py`, `backend/tests/test_widgets.py`, others as needed
+  - Lines: ~20
+  - Details: Remove `media_root`/`path_prefix` from machine fixtures.
+
+- [ ] **1.5 Verify backend**
+  - Run: `cd backend && .venv/bin/ruff check . && PYTHONPATH=src .venv/bin/python -m pytest`
+
+## Slice 2: Frontend cleanup
+
+**Goal:** Remove dead machine fields and the Jellyfin service option from Settings.
+
+- [ ] **2.1 Update types**
+  - Files: `frontend/src/types/index.ts`
+  - Lines: ~10
+  - Details: Remove `media_root` and `path_prefix` from `MonitoringMachine` and `MonitoringMachineInput`.
+
+- [ ] **2.2 Update Settings form**
+  - Files: `frontend/src/pages/Settings.tsx`
+  - Lines: ~60
+  - Details: Remove `media_root` and `path_prefix` inputs, remove `jellyfin` from the services multi-select options, update `emptyMachine()` and edit reset logic.
+
+- [ ] **2.3 Update frontend tests**
+  - Files: `frontend/src/pages/__tests__/Settings.test.tsx` if it exists, or other fixtures
+  - Lines: ~15
+  - Details: Remove fields from mock machines.
+
+- [ ] **2.4 Verify frontend**
+  - Run: `cd frontend && npm run lint && npm run build && npm run test`
+
+## Slice 3: Docs + changelog
+
+**Goal:** Document the completed migration and removed fields.
+
+- [ ] **3.1 Update REQUIREMENTS.md**
+  - Files: `docs/REQUIREMENTS.md`
+  - Lines: ~10
+  - Details: Remove references to machine-level `media_root`/`path_prefix` and Jellyfin service tagging.
+
+- [ ] **3.2 Update CHANGELOG.md**
+  - Files: `CHANGELOG.md`
+  - Lines: ~10
+  - Details: Add a breaking/changed note: machine config no longer includes `media_root`, `path_prefix`, or `jellyfin` in `services`; Jellyfin is configured exclusively via Services.
+
+- [ ] **3.3 Commit**
+  - Commit message: `refactor(settings): complete Jellyfin migration onto service registry`
+
+## Acceptance
+
+- `grep -R "media_root\|path_prefix" backend/src frontend/src` returns only `path_utils.py` and tests if retained, or nothing if deleted.
+- `jellyfin` is not offered as a machine service in the Settings UI.
+- Backend `pytest` and frontend `npm run lint/build/test` all pass.
+- Docs and changelog updated.