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/ai/providers/alibaba.py

@@ -1,6 +1,6 @@
 from typing import Dict, Any, Optional
 from aliyunsdkcore.client import AcsClient
-from aliyunsdkcore.auth.credentials import StsTokenCredential
+from aliyunsdkcore.auth.credentials import AccessKeyCredential
 from aliyunsdkalimt.request.v20181012 import TranslateGeneralRequest, TranslateECommerceRequest
 from app.services.translation_quota import TranslationQuotaService
 from app.database import AsyncSessionLocal