FOLLOW-UP: Make MaxFileSize configurable via generator flag

[adnaan]

Context

This follow-up task was identified during the review of PR #267.

Source PR: #267
PR Title: feat: add file upload & storage support (Roadmap 2.2)
Suggested by: @claude[bot]

Task Description

The MaxFileSize in the generated WithUpload config is hardcoded to 10 << 20 (10 MB). This will be wrong for many use cases (e.g., video uploads need more, avatar thumbnails need less).

Options:

• Add a --max-file-size flag to lvt gen resource
• Make it a named constant at the top of the generated handler
• Allow per-field size limits in the field spec (e.g., photo:image:5mb)

The simplest approach is a generator flag that sets the value for all file fields in that resource.

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This issue was automatically created by prmonitor from PR review comments.