Add db export malware warning

[musharafmaqbool]

What does it do?

Adds a warning message to the database export functionality to inform users that malware samples are not included in database exports since they are stored on the filesystem rather than in the database.

Changes:

• Added warning message to inform users about malware sample exclusion
• Warning appears when users access the export function
• Helps set proper expectations about export contents

Fixes: #3101

Testing

• Warning message appears when accessing DB export
• Message clearly explains malware samples are not included
• Export functionality still works normally

Questions

• Does it require a DB change? No
• Are you using it in production? This is a user-facing warning improvement
• Does it require a change in the API (PyMISP for example)? No

[musharafmaqbool]

Hi @adulau,

Thank you for the feedback. You're absolutely right - I understand now that each fix should be in its own separate branch with its own PR.

I've just created separate branches for each fix:

1. DB Export Malware Warning - This branch only contains the warning message for database exports
2. NIDS Email Duplicate Fix - I'm creating this as a separate branch that only addresses the duplicate email rules issue

I apologize for initially combining both changes in one PR. I'll make sure to follow the one-fix-per-PR approach going forward. Each branch now focuses on solving one specific issue with its own commit history.

Thank you for your patience and guidance on the proper contribution process.