Add PDF streaming viewer and token caching

[akashroy8210]

Changes

• Added backend PDF viewing endpoint.
• Streams PDFs from Microsoft Graph instead of redirecting users to the OneDrive preview page.
• Eliminates repeated OneDrive authentication prompts.
• Added access token caching to reduce request latency.
• Opens PDFs in a new browser tab using the browser's native PDF viewer.

Performance

Observed timings after caching:

• MongoDB lookup: ~5–15ms
• Access token retrieval: ~0–5ms (cached)
• Microsoft Graph fetch: ~700–800ms
• End-to-end PDF open time: typically under 1 second

User Impact

Users can now open PDFs directly in a new browser tab without being redirected to OneDrive or repeatedly authenticating.