Add file upload size validation

[meshackyaro]

Description

File upload endpoints accept files exceeding 10MB limit. DoS vector and resource waste.

Component

Backend

Difficulty

🟢 Easy

Tasks

• Enforce file size limits in multer config
• Validate size before processing
• Return clear error for oversized files
• Document upload limits
• Test with large files

Acceptance Criteria

• Oversized files rejected immediately
• Clear error message returned
• DoS vector mitigated

Estimated Time

1 day