chore(backlog): maintain task backlog and add changelog fragments

- Move completed tasks (85, 117, 118, 155) to backlog/completed/
- Delete superseded task files (166 verification, 172 drilldown)
- Add stats dashboard milestone m-1
- Add new tasks (190, 194)
- Update task metadata across remaining backlog items
- Add changelog fragments for stats, mpv args, and subtitle filtering

backlog/tasks/task-136 - Pin-SignPath-artifact-configuration-in-release-workflow.md

@@ -5,7 +5,7 @@ status: Done
 assignee:
   - codex
 created_date: '2026-03-08 20:41'
-updated_date: '2026-03-16 05:13'
+updated_date: '2026-03-18 05:28'
 labels:
   - ci
   - release
@@ -18,7 +18,7 @@ references:
   - build/signpath-windows-artifact-config.xml
   - src/release-workflow.test.ts
 priority: high
-ordinal: 48500
+ordinal: 49500
 ---
 
 ## Description