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/.pi-map.index.md

@@ -0,0 +1,21 @@
+# openspec/changes/jellyfin-service-registry (index)
+dir: openspec/changes/jellyfin-service-registry
+
+## role
+Documentation and planning package for migrating Jellyfin service configuration from machine-level settings to a service-registry model.
+## parent
+index: openspec/changes/.pi-map.index.md
+map: openspec/changes/.pi-map.md
+## children
+-
+## files
+- design.md
+- proposal.md
+- tasks.md
+## links
+index: openspec/changes/jellyfin-service-registry/.pi-map.index.md
+map: openspec/changes/jellyfin-service-registry/.pi-map.md
+## workflows
+-
+## dirty
+-