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.

internal/router/router.go

@@ -62,6 +62,7 @@ type Config struct {
 	LoginTracker          *ratelimit.LoginAttemptTracker
 	ScannerHandler        *handlers.Handler
 	JobsHandler           *handlers.JobsHandler
+	SidecarHandler        *handlers.SidecarHandler
 }
 
 // createJWTMiddleware creates a JWT middleware with proper user context setup
@@ -203,6 +204,7 @@ func RegisterRoutes(cfg *Config) *handlers.Handler {
 	registerAuthRoutes(cfg, rateLimitMiddleware)
 	registerLibraryRoutes(cfg)
 	registerDeviceRoutes(cfg)
+	registerSystemRoutes(cfg)
 	registerSyncRoutes(cfg)
 	registerCollectionsRoutes(cfg)
 	registerDashboardRoutes(cfg)