feat: Add email notifications for all notification types

Sends HTML email via Gmail SMTP when notifications are created.
Supports multi-language (en/ko/mn/ru) based on user country.
Runs in background thread to avoid blocking requests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/notification.py

@@ -10,6 +10,7 @@ from ..schemas.notification import (
     NotificationCreate, NotificationResponse,
     NotificationListResponse, NotificationMarkRead
 )
+from ..services.email_service import send_notification_email
 from .auth import get_current_user
 
 router = APIRouter(prefix="/notifications", tags=["notifications"])
@@ -29,7 +30,7 @@ def create_notification(
     related_id: Optional[int] = None,
     related_type: Optional[str] = None
 ) -> Notification:
-    """Create a new notification"""
+    """Create a new notification and send email"""
     notification = Notification(
         user_id=user_id,
         notification_type=notification_type,
@@ -42,6 +43,23 @@ def create_notification(
     db.add(notification)
     db.commit()
     db.refresh(notification)
+
+    # Send email notification (background thread, non-blocking)
+    try:
+        user = db.query(User).filter(User.id == user_id).first()
+        if user and user.email:
+            send_notification_email(
+                to_email=user.email,
+                notification_type=notification_type,
+                title=title,
+                message=message,
+                link=link,
+                user_name=user.name,
+                user_country=user.country,
+            )
+    except Exception as e:
+        print(f"[WARN] Email notification failed for user {user_id}: {e}")
+
     return notification