Create new shared package for Better Auth type definitions:
- UserRole, JWTPayload, CurrentUserData types
- Type guards: isValidUserRole, isValidOrganizationRole
- Utility functions: jwtPayloadToCurrentUser
- userAdditionalFields for client type inference
Migrate shared packages to use centralized types:
- @manacore/shared-auth re-exports from new package
- @manacore/shared-nestjs-auth uses new package as dependency
- Create @manacore/shared-nestjs-auth package with JwtAuthGuard
- Update @mana-core/nestjs-integration to validate tokens via auth service
- Replace insecure local JWT decode with server-side validation
- Integrate Zitare, Presi, ManaDeck backends with centralized auth
- Add DEV_BYPASS_AUTH support for development mode
- Document auth architecture in CLAUDE.md
