feat(api): add inbox-scoped public API for conversations, messages, saved replies, and teams

[realcodesiman]

Summary

• Adds workspace-token endpoints under the inbox API scope: conversation actions (assign, archive/unarchive, read/unread, follow/unfollow, enable/disable bot), conversation-scoped messages (list/get/create/edit/delete/attributes), saved replies CRUD, and enterprise team CRUD/membership.
• Every new handler delegates to the same business-layer service method the private UI action already calls, keeping the public and private paths consistent.
• Fixes conversationService.updateAssignment's bulk WHERE clause, which was missing a workspaceId scope — an oversight that would have made the new public assignment endpoint a cross-tenant write.

Changes

• apps/builder/src/features/conversations/api/public.ts, schema/public.ts — conversation action endpoints
• apps/builder/src/features/messages/api/public.ts, schema/public.ts — message CRUD endpoints (new)
• apps/builder/src/features/saved-replies/api/public.ts — saved reply CRUD endpoints
• apps/builder/src/enterprise/features/inbox-teams/api/public.ts — team CRUD/membership endpoints
• apps/builder/src/routers/public.ts — registers the new messages public router
• packages/business/src/conversation/service.ts — scopes updateAssignment's WHERE by workspaceId
• packages/business/src/enterprise/inbox-team/service.ts — create now returns the created team
• packages/business/src/saved-reply/service.ts — adds findByIdOrFail
• docs/developer/workspace-api-tokens.md — documents the inbox scope endpoint-to-scope table and invariants
• Test coverage: new __tests__/conversations-public-api.test.ts, conversations-public-scope.test.ts, inbox-teams-public-api.test.ts, messages-public-api.test.ts, saved-replies-public-api.test.ts

Test plan

• pnpm lint
• pnpm --filter builder check-types
• pnpm --filter builder vitest run on the new/changed test files (58 passed)
• pnpm --filter @chatbotx.io/business test for conversation/saved-reply/inbox-team services (2144 passed)
• Manual verification against a real workspace API token

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