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/state.rs

@@ -70,3 +70,13 @@ impl FromRef<AppState> for erp_workflow::WorkflowState {
         }
     }
 }
+
+/// Allow erp-message handlers to extract their required state without depending on erp-server.
+impl FromRef<AppState> for erp_message::MessageState {
+    fn from_ref(state: &AppState) -> Self {
+        Self {
+            db: state.db.clone(),
+            event_bus: state.event_bus.clone(),
+        }
+    }
+}