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-server/migration/src/lib.rs

@@ -22,6 +22,9 @@ mod m20260412_000019_create_process_instances;
 mod m20260412_000020_create_tokens;
 mod m20260412_000021_create_tasks;
 mod m20260412_000022_create_process_variables;
+mod m20260413_000023_create_message_templates;
+mod m20260413_000024_create_messages;
+mod m20260413_000025_create_message_subscriptions;
 
 pub struct Migrator;
 
@@ -51,6 +54,9 @@ impl MigratorTrait for Migrator {
             Box::new(m20260412_000020_create_tokens::Migration),
             Box::new(m20260412_000021_create_tasks::Migration),
             Box::new(m20260412_000022_create_process_variables::Migration),
+            Box::new(m20260413_000023_create_message_templates::Migration),
+            Box::new(m20260413_000024_create_messages::Migration),
+            Box::new(m20260413_000025_create_message_subscriptions::Migration),
         ]
     }
 }