[SECURITY] Add file upload size limits and validation

[mkilijanek]

Description

Currently, there is no limit on uploaded file sizes, which could lead to resource exhaustion attacks.

Requirements

• Add configurable max file size limit (default: 10MB)
• Validate file type before processing
• Add rate limiting for API endpoints to prevent abuse
• Return appropriate HTTP 413 for oversized files

Acceptance Criteria

• MAX_FILE_SIZE env var (default 10MB)
• File type validation (magic numbers)
• Rate limiting (flask-limiter)
• Tests for oversized files
• Documentation updated

References

• src/app.py: submit_analysis() endpoint