refactor: replace mileage monolith with modular route files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/server/index.ts

@@ -4,7 +4,7 @@ import { Hono } from 'hono';
 import { cors } from 'hono/cors';
 import dotenv from 'dotenv';
 import vehiclesRouter from './routes/vehicles.js';
-import mileageRouter from './routes/mileage.js';
+import mileageRouter from './routes/mileage/index.js';
 
 dotenv.config();