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.

packages/ui/src/index.ts

@@ -14,6 +14,7 @@ export { Skeleton } from "./skeleton";
 export { Label } from "./label";
 export { Separator } from "./separator";
 export { Tabs, TabsList, TabsTrigger, TabsContent } from "./tabs";
+export { Accordion, AccordionItem, AccordionTrigger, AccordionContent } from "./accordion";
 export {
   Dialog,
   DialogPortal,