router: register SidecarHandler routes for system config and device sidecar

Phase 1: Register routes that were defined but never connected

- Add GET/PUT /api/system/config routes (admin-only) for system
  configuration in new internal/router/system.go
- Add GET /api/devices/:id/sidecar routes for device sidecar config
- Add SidecarHandler to router Config struct
- Instantiate SidecarHandler in main.go with config for fallback support

These routes were implemented in handlers/sidecar.go but never registered,
breaking the ability to configure base URLs for device sync.

cmd/server/main.go

@@ -90,6 +90,7 @@ func main() {
 
 	authHandler := handlers.NewAuthHandler(queries, cfg.JWTSecret, loginAttemptTracker)
 	systemSettingsHandler := handlers.NewSystemSettingsHandler(queries)
+	sidecarHandler := handlers.NewSidecarHandler(queries, cfg)
 	libraryHandler := handlers.NewLibraryHandler(queries)
 	deviceHandler := handlers.NewDeviceHandler(queries, cfg.JWTSecret, cfg)
 	deviceAuthMiddleware := middleware.NewDeviceAuthMiddleware(queries)
@@ -182,6 +183,7 @@ func main() {
 		OPDSHandler:           opdsHandler,
 		Worker:                worker,
 		SystemSettingsHandler: systemSettingsHandler,
+		SidecarHandler:        sidecarHandler,
 		ConnManager:           connManager,
 		QueueProcessor:        queueProcessor,
 		DeviceAuthMiddleware:  deviceAuthMiddleware,