Add session mode for pre-funded request processing

Implement session-based API endpoints for creating, managing, and interacting with pre-funded sessions, including deposit and top-up invoice generation, macaroon authentication, and per-request debiting of compute costs.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 418bf6f8-212b-4bb0-a7a5-8231a061da4e
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Replit-Commit-Event-Id: 2dc3847e-7186-4a22-9c7e-16cd31bca8d9
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/routes/index.ts

@@ -2,6 +2,7 @@ import { Router, type IRouter } from "express";
 import healthRouter from "./health.js";
 import jobsRouter from "./jobs.js";
 import bootstrapRouter from "./bootstrap.js";
+import sessionsRouter from "./sessions.js";
 import demoRouter from "./demo.js";
 import devRouter from "./dev.js";
 import testkitRouter from "./testkit.js";
@@ -12,6 +13,7 @@ const router: IRouter = Router();
 router.use(healthRouter);
 router.use(jobsRouter);
 router.use(bootstrapRouter);
+router.use(sessionsRouter);
 router.use(demoRouter);
 router.use(testkitRouter);
 router.use(uiRouter);