feat: add periodic database cleanup every 6 hours (TDD)

- Cleans expired verifications and orphaned usage rows
- Previously only ran once on startup (13d+ uptime = accumulation)
- Interval uses .unref() to not block graceful shutdown
- Stopped during shutdown before pool.end()
- Idempotent start (safe to call multiple times)
- 6 TDD tests added (periodic-cleanup.test.ts)
- 663 tests total, all passing

src/utils/periodic-cleanup.ts

@@ -0,0 +1,30 @@
+import { cleanupStaleData } from "../services/db.js";
+import logger from "../services/logger.js";
+
+export const CLEANUP_INTERVAL_MS = 6 * 60 * 60 * 1000; // 6 hours
+
+let intervalHandle: ReturnType<typeof setInterval> | null = null;
+
+export function startPeriodicCleanup(): void {
+  // Idempotent — don't create duplicate intervals
+  if (intervalHandle !== null) return;
+
+  intervalHandle = setInterval(async () => {
+    try {
+      logger.info("Running periodic database cleanup...");
+      await cleanupStaleData();
+    } catch (err: unknown) {
+      logger.error({ err }, "Periodic database cleanup failed (non-fatal)");
+    }
+  }, CLEANUP_INTERVAL_MS);
+
+  // Don't prevent graceful shutdown
+  intervalHandle.unref();
+}
+
+export function stopPeriodicCleanup(): void {
+  if (intervalHandle !== null) {
+    clearInterval(intervalHandle);
+    intervalHandle = null;
+  }
+}