Add payment hash to job creation response in stub mode

Include the `paymentHash` in the `evalInvoice` object when creating a job in stub mode via `POST /api/jobs` to ensure the frontend receives it correctly.

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artifacts/api-server/src/routes/jobs.ts

@@ -215,7 +215,11 @@ router.post("/jobs", async (req: Request, res: Response) => {
 
     res.status(201).json({
       jobId,
-      evalInvoice: { paymentRequest: lnbitsInvoice.paymentRequest, amountSats: evalFee },
+      evalInvoice: {
+        paymentRequest: lnbitsInvoice.paymentRequest,
+        amountSats: evalFee,
+        ...(lnbitsService.stubMode ? { paymentHash: lnbitsInvoice.paymentHash } : {}),
+      },
     });
   } catch (err) {
     const message = err instanceof Error ? err.message : "Failed to create job";