Document notes analyzer input limits

[MyouzzZ]

Summary

• Add docs/notes-input-limits.md documenting notes analyzer request limits.
• Explain current minimum/maximum lengths, too-short and too-long behavior, PDF truncation, fallback behavior, and privacy notes.
• Include future improvement ideas and a pre-PR checklist for input-limit changes.

Closes #17.

Validation

• Documentation-only change; no runtime tests required.
• git diff --check -- docs/notes-input-limits.md
• Checked limits and behavior against backend/main.py and study-copilot-full.html.