Add file check middleware

[AmirAgassi]

Checklist

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Description

Adds middleware to validate file upload sizes with configurable min/max limits.

Linked Issues

• Fixes [BE Feat] Create file size check middleware #211

Type of Change

• New feature (non-breaking change which adds functionality).

Testing

• backend/internal/middleware/upload_test.go

[juancwu]

I would change the file name to file_size_check instead of upload, but anything else is lgtm

[aidantrabs]

lgtm