security: Protect Screenshot Endpoint

[nik-kale]

Summary

This PR removes public screenshot access and implements a protected endpoint that validates mission ownership before serving screenshots. Prevents unauthorized access to potentially sensitive dashboard screenshots.

Changes

• Removed public static serving of /screenshots directory
• Added protected endpoint: GET /api/screenshots/:missionId/:filename
• Mission ownership validation: Verifies screenshot belongs to mission
• Directory traversal protection: Validates file paths
• File existence checks: Returns 404 for missing screenshots
• Updated screenshot URLs: Include missionId in path

Type of Change

• Security patch
• New feature
• Bug fix
• Breaking change
• Documentation update

Security Improvements

• ✅ Screenshots no longer publicly accessible
• ✅ Mission ownership validated before serving
• ✅ Directory traversal attacks prevented
• ✅ File path sanitization
• ✅ Proper error handling (no path leakage)

API Changes

Before

GET /screenshots/screenshot-123.png  (public, no auth)

After

GET /api/screenshots/mission-123/screenshot-123.png  (requires mission to exist)

Testing

• ✅ No linting errors
• ✅ Screenshot endpoint validates mission exists
• ✅ Returns 404 for non-existent missions
• ✅ Returns 403 for screenshots not in mission
• ✅ Directory traversal blocked
• ✅ File existence checked

Checklist

• Code follows project style guidelines
• Self-review completed
• Security validation implemented
• No new warnings introduced
• Path sanitization verified

Related Issues

Addresses Feature #9 from FEATURE_OPPORTUNITIES.md - Priority Score: 2.0

Notes

• Screenshots now require knowing the missionId
• Frontend automatically uses new endpoint format
• Backward compatibility: old /screenshots URLs will 404

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[nik-kale]

Closing PR - screenshot protection implemented but reverted by user preference. Available for reference.