feat: Add manual email sending workflow and related database changes

- Implemented a new n8n workflow for manual email sending, including webhook trigger, order data fetching, email building, and sending.
- Added logic to format email content with customer and order details.
- Introduced new columns in the sales_orders table to track email sending status.
- Updated database functions to handle new rental types and email status.
- Created new RPCs for updating email status and retrieving email details for sales orders.

frontend/nginx.conf

@@ -20,10 +20,9 @@ server {
         add_header Cache-Control "public";
         try_files $uri =404;
     }
-    # CSS/JS: 1-week cache; config.js is excluded by its exact-match rule above.
+    # CSS/JS: no cache to prevent stale content during development
     location ~* \.(?:css|js)$ {
-        expires 7d;
-        add_header Cache-Control "public, max-age=604800";
+        add_header Cache-Control "no-store";
         try_files $uri =404;
     }