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/system.go

@@ -0,0 +1,20 @@
+package router
+
+import (
+	"bookhoard/internal/handlers"
+)
+
+func registerSystemRoutes(cfg *Config) {
+	e := cfg.Echo
+
+	// JWT middleware
+	jwtMiddleware := createJWTMiddleware(cfg)
+
+	// Protected routes (admin-only)
+	protected := e.Group("/api", jwtMiddleware)
+	system := protected.Group("/system", handlers.AdminMiddleware)
+
+	// System configuration routes (admin-only)
+	system.GET("/config", cfg.SidecarHandler.GetSystemConfiguration)
+	system.PUT("/config", cfg.SidecarHandler.UpdateSystemConfiguration)
+}