Feature/backend/delete entire collection

[Vriti62]

Closes: #1216
Closes: #1180

Describe the add-ons or changes you've made 📃

• Implemented an authenticated DELETE endpoint to remove a collection by its ID.
• Ensured cascade deletion: all projects inside the collection are deleted automatically using Mongoose middleware.
• Added owner validation so that only the collection owner (via userID) can delete it.
• Improved error handling with proper status codes and structured JSON responses.

Screenshots 📷

Type of change ☑️

What sort of change have you made:

• [x ] New feature
• [x ] Database update

Always open for any feedbacks!!

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