Implement API to Send Collaboration Invite for a Project (Backend)

[vansh909]

Description:
Implement a backend endpoint that allows a project owner to send a collaboration invite to users (via user ID, email, or username).

Must update the project schema with a pending invite.

Should validate against duplicate invites.
refers: #1179

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[Avdhesh-Varshney]

@vansh909 Not only for too much small part. Plz go with creating multiple api end-points
Then raise multiple sub-issues like 1 sub-issue for 1 api end-point

Plz update this issue title and desc accordingly.

Don't raise for schema then model, etc.