Fix : add max file size validation

[smitdodiya]

Issue:
Uploading large quiz files causes unexpected errors instead of showing a clear validation message. (#59 )

Problem:
When a file larger than the allowed size is uploaded, the request may fail with 413 Request Entity Too Large and CORS-related errors. This prevents users from receiving a friendly and understandable error message for oversized file uploads.

What this PR does:

• Sets a 15 MB request body limit in the Fiber server.
• Adds a custom error message
  "The file is too large to upload. Please select a smaller file."
• Preserves the existing 1 MB application-level validation for quiz CSV uploads, ensuring consistent frontend error handling.