Backend : Delete Entire Collection with Projects

[Vriti62]

Description
This sub-issue focuses on allowing a logged-in user to delete an entire collection along with all the saved projects inside it.

Tasks / Requirements
-Add an authenticated DELETE endpoint to remove a collection by its ID.
-Ensure that all projects belonging to that collection are deleted (since each saved project is stored as a separate document).
-Validate that only the owner (userID) of the collection can delete it.
-Return success or appropriate error responses (e.g., collection not found).

refers: #1180

[github-actions]

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