feat: multi-browser pooling (2 Chromium instances × 8 pages)

- Launch BROWSER_COUNT separate Chromium instances (default: 2)
- Each with PAGES_PER_BROWSER pages (default: 8, 16 total)
- Round-robin distribution across browser instances
- Independent restart scheduling per browser
- Updated health endpoint to show per-browser stats
- docker-compose: added BROWSER_COUNT and PAGES_PER_BROWSER env vars

src/routes/health.ts

@@ -1,7 +1,21 @@
 import { Router } from "express";
+import { getPoolStats } from "../services/browser.js";
 
 export const healthRouter = Router();
 
 healthRouter.get("/", (_req, res) => {
-  res.json({ status: "ok", version: "0.1.0" });
+  const pool = getPoolStats();
+  res.json({
+    status: "ok",
+    version: "0.2.1",
+    pool: {
+      size: pool.poolSize,
+      active: pool.totalPages - pool.availablePages,
+      available: pool.availablePages,
+      queueDepth: pool.queueDepth,
+      pdfCount: pool.pdfCount,
+      restarting: pool.restarting,
+      uptimeSeconds: Math.round(pool.uptimeMs / 1000),
+    },
+  });
 });