Add PDF upload validation plan

[MyouzzZ]

Summary

• add a PDF upload validation plan covering accepted file rules, size limits, invalid PDF handling, and security/performance concerns
• document backend test cases for missing files, non-PDF uploads, oversized files, invalid bytes, empty PDFs, and valid text PDFs
• align the plan with the current backend .pdf filename check, 10 MB limit, and PyPDF2 extraction behavior

Closes #15.

Validation

• rg -n MAX_PDF_BYTES backend\main.py docs\pdf-upload-validation-plan.md
• rg -n "Only PDF files are supported|Could not read PDF|No readable text|MAX_NOTES_CHARS|GOOGLE_API_KEY" backend\main.py docs\pdf-upload-validation-plan.md
• git diff --check -- docs/pdf-upload-validation-plan.md