Backend: Save a New Project to an Existing Collection (or Auto create the collection)

[Vriti62]

Description

Currently, users don’t have the ability to add a project into their saved collection.
This feature will enable logged-in users to save a project (by projectId) into their existing collection of saved projects.

Tasks / Requirements
-Create an authenticated endpoint to save a project into a user’s existing collection.
-Ensure duplicate entries are prevented (a project should not be saved twice in the same collection).
-Use the user’s userId from the auth middleware to map the project to their saved collection.
-Return the updated saved collection in the response.
-if no collection of the name exists- it auto creates the collection with the name received

Why this is needed
-This feature lets users organize and revisit projects easily, making collections truly functional.
It’s the first step toward the complete Saved Projects with Sorting Options feature.

refer: #1180

[github-actions]

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