fix: implement pending room invitations instead of direct member addition

[codecrafted1]

📝 Description

This PR updates the room invitation flow to prevent users from being added to collaboration rooms immediately after entering their GitHub username.

Changes

• Added a createRoomInvitation() helper to create pending room invitations.
• Added a getPendingInvitation() helper to check for existing pending invitations.
• Updated the room invite API to create a pending invitation instead of directly adding the user to room_members.
• Added validation to prevent duplicate pending invitations.
• Preserved the existing checks for room ownership, valid GitHub usernames, GitHub account verification, and existing room members.

Why?

Previously, inviting a user immediately inserted them into the room_members table, making them a full room member without any confirmation or approval.

This change introduces the first step toward a consent-based invitation flow by storing invitations separately instead of granting immediate room access.

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🔗 Related Issue

Closes #3239

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🏷️ Type of Change

• 🐛 Bug fix
• ✨ Enhancement

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📋 Checklist

• Code follows the project guidelines.
• Existing validations have been preserved.
• Added duplicate pending invitation validation.
• Tested locally.

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Notes

This PR introduces pending room invitations as the backend foundation. Users are no longer added directly to a room when invited.

[github-actions]

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[codecrafted1]

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