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/shared/src/schemas/changelog.ts

@@ -0,0 +1,27 @@
+import { z } from "zod";
+
+export const changelogStageEnum = z.enum(["alpha", "beta", "prod"]);
+
+export const changelogEntrySchema = z.object({
+  id: z.string().uuid(),
+  stage: changelogStageEnum,
+  title: z.string().min(1).max(255),
+  description: z.string().min(1),
+  publishedAt: z.string().datetime(),
+  createdAt: z.string().datetime(),
+  updatedAt: z.string().datetime(),
+});
+
+export const createChangelogEntrySchema = z.object({
+  stage: changelogStageEnum,
+  title: z.string().min(1).max(255),
+  description: z.string().min(1),
+  publishedAt: z.string().datetime(),
+});
+
+export const updateChangelogEntrySchema = createChangelogEntrySchema.partial();
+
+export type ChangelogEntry = z.infer<typeof changelogEntrySchema>;
+export type CreateChangelogEntry = z.infer<typeof createChangelogEntrySchema>;
+export type UpdateChangelogEntry = z.infer<typeof updateChangelogEntrySchema>;
+export type ChangelogStage = z.infer<typeof changelogStageEnum>;