feat: add structured JSON logger and worker utility (P1-4)

Introduces src/lib/logger.ts emitting JSON lines (level, message, data)
compatible with Cloudflare Logpush. Replaces all console.log/warn/error
calls in email-processor.ts, index.ts, and hub.ts with structured logger
calls. Extracts waitUntilSafe into src/utils/worker.ts to avoid duplicating
the executionCtx guard across routes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/utils/worker.ts

@@ -0,0 +1,10 @@
+import { Context } from "hono";
+
+/** Calls ctx.waitUntil() without throwing when the ExecutionContext is absent (e.g. Node tests). */
+export function waitUntilSafe(c: Context, promise: Promise<unknown>): void {
+  try {
+    c.executionCtx.waitUntil(promise);
+  } catch {
+    // ExecutionContext unavailable in Node test environment — ignore.
+  }
+}