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/admin_search.py

@@ -181,9 +181,8 @@ async def test_provider(
         return {"success": False, "error": str(e)}
 
 
+from app.core.utils import validate_uuid
+
+
 def _validate_uuid(uuid_str: str):
-    import uuid
-    try:
-        uuid.UUID(uuid_str)
-    except ValueError:
-        raise HTTPException(status_code=400, detail="Invalid UUID")
+    validate_uuid(uuid_str)