fix: await flushDirtyEntries during shutdown to prevent usage data loss

Remove fire-and-forget SIGTERM/SIGINT handlers from usage.ts (race condition
with pool.end() in index.ts). Instead, await flushDirtyEntries() in the
index.ts shutdown orchestrator between stopping the server and closing the
DB pool.

src/index.ts

@@ -18,7 +18,7 @@ import { recoverRouter } from "./routes/recover.js";
 import { emailChangeRouter } from "./routes/email-change.js";
 import { billingRouter } from "./routes/billing.js";
 import { authMiddleware } from "./middleware/auth.js";
-import { usageMiddleware, loadUsageData } from "./middleware/usage.js";
+import { usageMiddleware, loadUsageData, flushDirtyEntries } from "./middleware/usage.js";
 import { getUsageStats } from "./middleware/usage.js";
 import { pdfRateLimitMiddleware, getConcurrencyStats } from "./middleware/pdfRateLimit.js";
 import { initBrowser, closeBrowser } from "./services/browser.js";
@@ -406,6 +406,14 @@ async function start() {
       });
     });
 
+    // 1.5. Flush dirty usage entries while DB pool is still alive
+    try {
+      await flushDirtyEntries();
+      logger.info("Usage data flushed");
+    } catch (err) {
+      logger.error({ err }, "Error flushing usage data during shutdown");
+    }
+
     // 2. Close Puppeteer browser pool
     try {
       await closeBrowser();