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/entity/message_subscription.rs

@@ -0,0 +1,31 @@
+use sea_orm::entity::prelude::*;
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Debug, PartialEq, DeriveEntityModel, Serialize, Deserialize)]
+#[sea_orm(table_name = "message_subscriptions")]
+pub struct Model {
+    #[sea_orm(primary_key, auto_increment = false)]
+    pub id: Uuid,
+    pub tenant_id: Uuid,
+    pub user_id: Uuid,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub notification_types: Option<serde_json::Value>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub channel_preferences: Option<serde_json::Value>,
+    pub dnd_enabled: bool,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub dnd_start: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub dnd_end: Option<String>,
+    pub created_at: DateTimeUtc,
+    pub updated_at: DateTimeUtc,
+    pub created_by: Uuid,
+    pub updated_by: Uuid,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub deleted_at: Option<DateTimeUtc>,
+}
+
+#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
+pub enum Relation {}
+
+impl ActiveModelBehavior for ActiveModel {}