feat(FN-188): add Changelog tab to dashboard settings

Squash-recovery of fusion/fn-188 onto dev — original branch was based on
main (dccd4fa) due to baseBranch=None drift in Fusion settings; rebase
onto dev surfaced unrelated main-only commits (EMEX/PL24/Corgi) as false
conflicts. This commit applies only FN-188's 23-file changelog patch.

- apps/api: ChangelogModule (controller, service, DTO, spec, schema)
- apps/web: ChangelogTab (timeline + accordion), useChangelog hook, i18n
- packages/shared: changelog Zod schemas + types
- packages/ui: Accordion component + Badge stage variant
- docs/INDEX.md: changelog feature documented

Lint, typecheck, all 169 api tests + 2 web tests pass.

apps/web/src/routes/__tests__/index.test.tsx

@@ -6,6 +6,7 @@
  */
 
 import { fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { BrowserRouter } from "react-router-dom";
 import { vi } from "vitest";
 
 // Mock the PostHog capture function