Backend: Create Collection Endpoint (User Option + Auto-Creation)

[Vriti62]

Description
This sub-issue focuses on creating a new collection for saved projects.
A user should be able to manually create a new collection by providing a name.
If no collection exists when the user tries to save a project, the system should automatically create a default collection (e.g., “My Saved Projects”).

Tasks / Requirements
-Add an authenticated POST endpoint to create a new collection.
-Validate so that duplicate collection names for the same user are not created.
-Auto-create a default collection if none exists when saving a project - taken care of in issue #1194

refers: #1180

[github-actions]

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[Avdhesh-Varshney]

@Vriti62 Nice to consider this edge case 👍