Add content-length validation and file type enforcement for media uploads #850

Overview

src/workers/processors/media-processing.worker.ts and the CDN upload endpoint do not validate file size limits or enforce allowed MIME types beyond what the client declares in the Content-Type header. An attacker can upload arbitrarily large files or files with disguised MIME types (e.g. a PHP script named video.mp4).

Specifications

Features:

• Enforce max file size (configurable, default 500MB for video, 10MB for images).
• Validate actual file magic bytes against the declared MIME type.

Tasks:

• Add multer limits (fileSize) or manual Content-Length check in the upload endpoint.
• Use file-type package to inspect magic bytes and compare to expected MIME.
• Reject mismatched files with 415 Unsupported Media Type.
• Add unit tests for oversized files and MIME mismatch.

Impacted Files:

• src/cdn/cdn.service.ts
• src/cdn/dto/upload-content.dto.ts

Acceptance Criteria

• Upload of a file exceeding the size limit returns 413.
• File with wrong magic bytes returns 415.
• Valid uploads continue to work.