fix: security and code quality improvements

Security fixes:
- Add file upload size limits (10MB) for customer and product imports
- Add XLSX file validation with row limits and magic byte checking
- Implement password validation (min 6 chars) in registration
- Add rate limiting for guest login (5 per IP per 15 minutes)
- Sanitize error messages to prevent information leakage
- Fix XSS vulnerability by removing unsafe v-html usage
- Enforce WhatsApp webhook signature verification
- Add SSRF protection with URL validation and IP blocking
- Fix marketing endpoints to use proper authentication

Code quality improvements:
- Create shared utility functions for UUID validation and string sanitization
- Remove duplicate UUID validation code from admin modules
- Remove dead code (pass statement in translation.py)
- Fix aliyun SDK import compatibility

backend/app/api/v1/whatsapp.py

@@ -38,6 +38,8 @@ async def handle_webhook(
     if x_hub_signature_256:
         if not svc.verify_signature(body, x_hub_signature_256):
             raise HTTPException(status_code=403, detail="Invalid signature")
+    else:
+        raise HTTPException(status_code=403, detail="Missing signature")
 
     import json
     body_json = json.loads(body)