Check Paper in DB before submitting request paper and also check paperRequest collection to check already user has requested or not

[YOGESH-08]

Is your feature request related to a problem? Please describe.

Yes. Currently, the "Request Paper" functionality does not check if a paper already exists in the database before allowing a user to submit a request. This leads to:

1. Redundant Data: Duplicate requests for the same paper are stored in the database.
2. Poor User Experience: Users might request a paper that is already available on the platform without realizing it.
3. Manual Overload: Admins have to manually filter out requests for papers they already have.

Describe the solution you'd like

Implement a validation layer in the paper request workflow:

1. Server-side Validation: Update src/app/api/request/route.ts to check Paper and Admin matching collections (subject, exam, slot, year) before creating a request.
2. Duplicate Prevention: Prevent the creation of a new PaperRequest if an identical one already exists.
3. Graceful Error Handling: Return specific status codes (e.g., 409 Conflict) so the frontend can distinguish between "already exists in library" and "already requested".
4. Enhanced UI: Update RequestModal and the PaperRequest page to show helpful messages and a "View Paper" button if it's already available.

Additional context

• Relevant files: src/app/api/request/route.ts, src/app/request/page.tsx, src/components/ui/RequestModal.tsx.
• Database Models: Paper, PaperAdmin, PaperRequest.
• Goals: Help maintain a clean database and improve user journey.

[YOGESH-08]

but if these checks are done in papers main, then it would affect ux
better to handle at admin portal