perf: add mongodb indexes to prevent dashboard collscan

[Unnati1007]

Description

This PR resolves the severe latency and CPU load issue caused by unindexed queries in the /dashboard/history and /dashboard/stats backend endpoints.

• Added an async MongoDB index creation block to the FastAPI lifespan startup event in backend/main.py.
• Created a compound index on user_email (ascending) and date (descending) for the problem_info collection to ensure instant pagination.
• Created a unique index on email (ascending) for the users collection to guarantee fast lookups.
• Resolved IDE and runtime initialization scoping warnings by properly hoisting mongo_client and db.
• Restored missing critical imports (PlatformCredential, encrypt, resolve_user_credentials) to prevent runtime crashes during authentication.

Related Issue #185

Type of Change

• Bug fix
• New feature (Performance Optimization)
• Documentation update

Checklist

• My code follows project guidelines
• I have tested my changes
• Documentation updated if needed

[Unnati1007]

@vanshaggarwal27 please review the pr and assign me the issue i raised