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/__tests__/usage-shutdown.test.ts

@@ -0,0 +1,30 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+vi.unmock("../middleware/usage.js");
+
+describe("usage shutdown race condition fix", () => {
+  it("should NOT register SIGTERM/SIGINT handlers as module-level side effects", async () => {
+    const onSpy = vi.spyOn(process, "on");
+    vi.resetModules();
+    
+    // Track which signals usage.ts registers
+    const signalsBefore = onSpy.mock.calls.map(c => c[0]);
+    
+    await import("../middleware/usage.js");
+    
+    const signalsAfter = onSpy.mock.calls.map(c => c[0]);
+    const newSignals = signalsAfter.slice(signalsBefore.length);
+    
+    // usage.ts should NOT register SIGTERM or SIGINT handlers
+    const usageSignals = newSignals.filter(s => s === "SIGTERM" || s === "SIGINT");
+    expect(usageSignals).toEqual([]);
+    
+    onSpy.mockRestore();
+  });
+
+  it("should export flushDirtyEntries for external shutdown orchestration", async () => {
+    vi.resetModules();
+    const usageMod = await import("../middleware/usage.js");
+    expect(typeof usageMod.flushDirtyEntries).toBe("function");
+  });
+});