feat: prepare managed demo workflow and scenario docs

[technowizard]

Summary

Prepares Trestle for a hosted synthetic invoice-processing demonstration with managed services, complete workflow coverage, and user-facing demo documentation.

Changes

• Add Google AI Studio extraction with shared model adapter contracts and schema-level evidence defaults.
• Support Neon/R2 configuration, unique per-upload document keys, seeded reference data, and administrator demo reset in private mode.
• Load approval policy and purchase-order references during worker validation so Finance approval scenarios work end to end.
• Improve invoice action feedback with loading states, disabled controls during mutations, and clearer intake completion behavior.
• Add synthetic invoice PDFs covering clean processing, high-value approval, duplicates, missing PO, and currency mismatch.
• Reorganize the README and add DEMO_GUIDE.md with local, managed-service, hosted, and scenario walkthroughs.

Verification

• Commit hooks passed Oxlint and Oxfmt.
• Web test suite passed: 47 tests.
• Focused server tests passed for Google extraction, R2 storage, and invoice validation.
• Documentation links, formatting, and diff checks passed.