feat: persist rate overrides to DB when applying prices in AvailabilityPage

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

backend/src/app.ts

@@ -26,6 +26,7 @@ import rentalRoutes        from './routes/rental'
 import categoriesRoutes    from './routes/categories'
 import tariffsRoutes       from './routes/tariffs'
 import ratePeriodsRoutes   from './routes/rate-periods'
+import rateOverridesRoutes from './routes/rate-overrides'
 import uploadRoutes        from './routes/upload'
 
 export async function buildApp() {
@@ -97,6 +98,7 @@ export async function buildApp() {
   await fastify.register(categoriesRoutes)
   await fastify.register(tariffsRoutes)
   await fastify.register(ratePeriodsRoutes)
+  await fastify.register(rateOverridesRoutes)
   await fastify.register(uploadRoutes)
 
   return fastify