feat: document API schema in OpenAPI/Swagger (#507)

[diyasharma22]

Closes #507

What was changed

backend/app/main.py

• Added rich app description with Rate Limiting and Auth sections
• Added contact, license_info, and full openapi_tags list
• Added summary and description to /, /health, /ping endpoints

backend/app/schemas.py

• Added Field(description=, example=) to every field on every model
• Documented all request/response models including auth, share, chat, history

backend/app/routers/debugging.py

• Added summary, description, and responses block (200, 422, 429, 500)

How to verify

1. cd backend && uvicorn app.main:app --reload
2. Open http://localhost:8000/docs
3. Every endpoint grouped by tag, every field shows description + example
4. pytest -v — all 52 tests pass

Type of change

• Documentation update