Add honest accounting and automatic refund mechanism for completed jobs

Implement honest accounting post-job completion, calculating actual costs, adding margin, and enabling automatic refunds for overpayments via a new endpoint.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 418bf6f8-212b-4bb0-a7a5-8231a061da4e
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Event-Id: c6386de2-d5f4-47cc-a557-73416f09e118
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/9f85e954-647c-46a5-90a7-396e495a805a/418bf6f8-212b-4bb0-a7a5-8231a061da4e/sPDHkg8
Replit-Helium-Checkpoint-Created: true

artifacts/api-server/src/lib/pricing.ts

@@ -158,6 +158,35 @@ export class PricingService {
     const amountSats = usdToSats(estimatedCostUsd, btcPriceUsd);
     return { amountSats, estimatedCostUsd, marginPct: this.marginPct, btcPriceUsd };
   }
+
+  // ── Post-work honest accounting ──────────────────────────────────────────
+
+  /**
+   * Full actual charge in USD: raw Anthropic token cost + DO infra amortisation + margin.
+   * Pass in the already-computed actualCostUsd (raw token cost, no extras).
+   */
+  calculateActualChargeUsd(actualCostUsd: number): number {
+    const rawCostUsd = actualCostUsd + DO_INFRA_PER_REQUEST_USD;
+    return rawCostUsd * (1 + this.marginPct / 100);
+  }
+
+  /**
+   * Convert the actual charge to satoshis using the BTC price that was locked
+   * at invoice-creation time. This keeps pre- and post-work accounting in the
+   * same BTC denomination without a second oracle call.
+   */
+  calculateActualChargeSats(actualCostUsd: number, lockedBtcPriceUsd: number): number {
+    const chargeUsd = this.calculateActualChargeUsd(actualCostUsd);
+    return usdToSats(chargeUsd, lockedBtcPriceUsd);
+  }
+
+  /**
+   * Refund amount in sats: what was overpaid by the user.
+   * Always >= 0 (clamped — never ask the user to top up due to BTC price swings).
+   */
+  calculateRefundSats(workAmountSats: number, actualAmountSats: number): number {
+    return Math.max(0, workAmountSats - actualAmountSats);
+  }
 }
 
 export const pricingService = new PricingService();