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/src/config.rs

@@ -8,6 +8,7 @@ pub struct AppConfig {
     pub jwt: JwtConfig,
     pub auth: AuthConfig,
     pub log: LogConfig,
+    pub cors: CorsConfig,
 }
 
 #[derive(Debug, Clone, Deserialize)]
@@ -45,6 +46,13 @@ pub struct AuthConfig {
     pub super_admin_password: String,
 }
 
+#[derive(Debug, Clone, Deserialize)]
+pub struct CorsConfig {
+    /// Comma-separated list of allowed origins.
+    /// Use "*" to allow all origins (development only).
+    pub allowed_origins: String,
+}
+
 impl AppConfig {
     pub fn load() -> anyhow::Result<Self> {
         let config = config::Config::builder()