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.

CHANGELOG.md

@@ -99,9 +99,23 @@ All notable changes to Manage. Breaking changes are marked with **BREAKING**.
 
 ### Notes / follow-ups
 
-- Machine-level Jellyfin/Jellyseerr app config still powers the Media/Users/Files
-  pages. Migrating those onto the service registry is a separate follow-up change
-  (see `openspec/changes/service-registry/design.md` §12.5).
+- ~~Machine-level Jellyfin/Jellyseerr app config still powers the Media/Users/Files
+  pages. Migrating those onto the service registry is a separate follow-up change.~~
+  **Done (2026-06-23):** Jellyfin is no longer a machine service, and the dead
+  machine-level `media_root`/`path_prefix` fields were removed. See the
+  Jellyfin migration entry in `docs/REQUIREMENTS.md`.
+
+## Follow-up #2 — remove dead machine `media_root`/`path_prefix` + Jellyfin service
+
+Completes the Jellyfin migration onto the service registry. Jellyfin is no
+longer a machine `services` tag (`DEFAULT_SERVICES` is now `["monitoring",
+"files"]`), and the dead machine-level `media_root`/`path_prefix` fields were
+removed from the settings store, `MonitoringMachineInput`, frontend types, and
+the Settings UI. Jellyfin is configured exclusively as a service-registry
+instance. The global `REMOTE_MEDIA_ROOT`/`REMOTE_PATH_PREFIX` config properties
+and `path_utils.py` remain (files/media-index still use them for Jellyfin→SSH
+path resolution). Existing DB rows may still carry these keys in `config_json`;
+they are inert and get dropped on the next machine save.
 
 ## Follow-up #1 — remove dead machine Jellyfin/Jellyseerr fields