refactor: migrate HTTP routing to Hono framework

- Add hono dependency
- Replace if/else routing chain with Hono app
- Convert all HTTP routes to Hono format:
  - GET /health, /setup-webhook, /webhook-info
  - POST /webhook
  - ALL /api/*
  - GET /
- Keep email, scheduled, queue handlers unchanged
- Maintain 100% backward compatibility

Benefits:
- Cleaner declarative routing
- Type-safe Env bindings
- Ready for future middleware (CORS, rate limiting)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -32,6 +32,7 @@
     "ai"
   ],
   "dependencies": {
+    "hono": "^4.11.7",
     "zod": "^4.3.5"
   }
 }