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/m20260413_000023_create_message_templates.rs

@@ -0,0 +1,93 @@
+use sea_orm_migration::prelude::*;
+
+#[derive(DeriveMigrationName)]
+pub struct Migration;
+
+#[async_trait::async_trait]
+impl MigrationTrait for Migration {
+    async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .create_table(
+                Table::create()
+                    .table(MessageTemplates::Table)
+                    .if_not_exists()
+                    .col(
+                        ColumnDef::new(MessageTemplates::Id)
+                            .uuid()
+                            .not_null()
+                            .primary_key(),
+                    )
+                    .col(ColumnDef::new(MessageTemplates::TenantId).uuid().not_null())
+                    .col(
+                        ColumnDef::new(MessageTemplates::Name)
+                            .string()
+                            .not_null(),
+                    )
+                    .col(
+                        ColumnDef::new(MessageTemplates::Code)
+                            .string()
+                            .not_null(),
+                    )
+                    .col(
+                        ColumnDef::new(MessageTemplates::Channel)
+                            .string()
+                            .not_null()
+                            .default("in_app"),
+                    )
+                    .col(
+                        ColumnDef::new(MessageTemplates::TitleTemplate)
+                            .string()
+                            .not_null(),
+                    )
+                    .col(
+                        ColumnDef::new(MessageTemplates::BodyTemplate)
+                            .text()
+                            .not_null(),
+                    )
+                    .col(
+                        ColumnDef::new(MessageTemplates::Language)
+                            .string()
+                            .not_null()
+                            .default("zh-CN"),
+                    )
+                    .col(ColumnDef::new(MessageTemplates::CreatedAt).timestamp_with_time_zone().not_null())
+                    .col(ColumnDef::new(MessageTemplates::UpdatedAt).timestamp_with_time_zone().not_null())
+                    .col(ColumnDef::new(MessageTemplates::CreatedBy).uuid().not_null())
+                    .col(ColumnDef::new(MessageTemplates::UpdatedBy).uuid().not_null())
+                    .col(ColumnDef::new(MessageTemplates::DeletedAt).timestamp_with_time_zone().null())
+                    .to_owned(),
+            )
+            .await?;
+
+        manager.get_connection().execute(sea_orm::Statement::from_string(
+            sea_orm::DatabaseBackend::Postgres,
+            "CREATE UNIQUE INDEX idx_message_templates_tenant_code ON message_templates (tenant_id, code) WHERE deleted_at IS NULL".to_string(),
+        )).await.map_err(|e| DbErr::Custom(e.to_string()))?;
+
+        Ok(())
+    }
+
+    async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .drop_table(Table::drop().table(MessageTemplates::Table).to_owned())
+            .await
+    }
+}
+
+#[derive(DeriveIden)]
+enum MessageTemplates {
+    Table,
+    Id,
+    TenantId,
+    Name,
+    Code,
+    Channel,
+    TitleTemplate,
+    BodyTemplate,
+    Language,
+    CreatedAt,
+    UpdatedAt,
+    CreatedBy,
+    UpdatedBy,
+    DeletedAt,
+}