feat: add email notifications for review comments

[NagariaHussain]

Summary

• Notify the asset uploader when someone else comments on their upload
• Notify the parent comment author when someone replies to their comment
• Emails sent via background job (frappe.enqueue) to avoid blocking the API response
• Self-notifications are skipped (commenter == uploader or commenter == parent author)

Implementation

• New vms/notifications.py with send_comment_notification (hook) and _send_comment_email (worker)
• Wired via doc_events in hooks.py on VMS Review Comment.after_insert

Test plan

• Comment on asset where uploader is a different user → email queued and sent
• Reply to another user's comment → email queued and sent
• Comment on own upload → no email sent (self-notification skipped)
• Reply to own comment → no email sent
• Verified via Email Queue list view — all test emails show status "Sent"

Closes #3

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