feat(message): add message center module (Phase 5)
Implement the complete message center with:
- Database migrations for message_templates, messages, message_subscriptions tables
- erp-message crate with entities, DTOs, services, handlers
- Message CRUD, send, read/unread tracking, soft delete
- Template management with variable interpolation
- Subscription preferences with DND support
- Frontend: messages page, notification panel, unread count badge
- Server integration with module registration and routing
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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