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>

crates/erp-message/src/lib.rs

@@ -1 +1,10 @@
-// erp-message: 消息中心模块 (Phase 5)
+pub mod dto;
+pub mod entity;
+pub mod error;
+pub mod handler;
+pub mod message_state;
+pub mod module;
+pub mod service;
+
+pub use message_state::MessageState;
+pub use module::MessageModule;