feat: add QuickBooks mock connector contract

[lovewave02]

PR Draft: #6

Title:

feat: add QuickBooks mock connector contract

Body:

## Summary

- add a QuickBooks-specific mock connector contract to the accounting platform profile
- model vendor bill draft creation, vendor bill payload validation, and bill payment status sync fields
- keep the integration contract-only: no live QuickBooks calls, credentials, or money movement
- add focused coverage for the QuickBooks contract while preserving exact/generic profile behavior

Closes #6.

## Validation

- [x] `python3 -m py_compile app/services/accounting_platforms.py tests/test_accounting_platforms.py`
- [x] Direct Python assertions for existing exact/generic behavior and the new QuickBooks contract
- [x] `git diff --check`
- [ ] `uv run pytest` - not run locally because `uv` is not installed in this environment
- [ ] `uv run python -m compileall app tests scripts` - not run locally because `uv` is not installed in this environment

## Notes

This keeps QuickBooks as a mock connector contract only. It does not introduce OAuth, API credentials, live posting, or payment movement.