Add OpenBooks as open source alternative to QuickBooks

Using CONTRIBUTING_TO_OSB.md, please add:

Add open source alternative to QuickBooks:
- Name: OpenBooks
- Repository: https://github.com/braedonsaunders/openbooks
- Description: Accounting-first self-hosted ERP with a PostgreSQL-enforced double-entry ledger, multi-entity, job costing, invoices/bills, inventory, approvals, and audit trail (AGPL, Docker).
- Similarity Score: 70
- Notes: Stronger on mid-market / multi-entity and project job costing than classic QuickBooks Online. Alpha. Self-hosted only (no SaaS pitch).

Category: Finance & Accounting
Features: Double-entry ledger, multi-entity, job costing, invoicing, bills, inventory, approvals, audit trail, Docker self-host

Thanks!

Start with CONTRIBUTING_TO_OSB.md to follow the repository's process and locate the alternatives data or page to update. Add the supplied OpenBooks name, repository, description, score, notes, category, and features, then verify the entry appears under Finance & Accounting with the provided details.

Written by the indexing model from the issue text.