feat: add treasury liquidity controls

[OkeyAmy]

Summary

• Prevent payouts/refunds from consuming liquidity that is not immediately available by defining treasury buckets and deterministic solvency checks.
• Ensure all treasury math is reproducible, auditable, and uses integer minor units to avoid rounding ambiguity.

Description

• Add a lightweight treasury service with integer-minor-unit bucket definitions, liquidity/solvency calculation, and hold decision logic (lib/treasury/service.ts).
• Extend ledger account categories to include settlement_in_transit, refunds_payable, platform_reserve, and suspense, and add snapshot/adjustment models (models/TreasurySnapshot.ts, models/TreasuryAdjustmentProposal.ts, models/LedgerAccount.ts).
• Add admin-only APIs to compute on-demand treasury summaries and create reproducible daily snapshots (app/api/admin/treasury/summary/route.ts) and to propose audited treasury adjustments that never mutate user balances (app/api/admin/treasury/adjustments/route.ts).
• Add focused tests and documentation covering bucket definitions, reserve/concentration holds, snapshot reproducibility, and adjustment safeguards (__tests__/lib/treasury/service.test.ts, docs/treasury-controls.md).

Testing

• Ran npm run lint and it passed.
• Ran npm run typecheck and npm run typecheck:gate and both passed.
• Ran the new unit tests with npx vitest run __tests__/lib/treasury/service.test.ts and they passed.
• Ran contract tests with npm run test:contracts and the suite passed.
• npm run openapi:check failed due to an existing OpenAPI generation drift unrelated to the treasury changes (the generator flagged contract drift for pre-existing consent-related paths).
• npm run build did not complete successfully in this environment (the next build --webpack process was CPU-bound and was terminated), so a full production build should be verified in CI or locally.

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Close #69
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Close #69