feat: replace placeholder auth with db hash

cmd/server/main.go

@@ -126,6 +126,7 @@ func main() {
 	r.GET("/health", h.Health)
 	r.GET("/hello", h.Hello)
 	r.POST("/login", h.Login)
+	r.POST("/register", h.Register)
 	r.GET("/devices", h.GetDevices)
 
 	// mqttping endpoint handled by internal/handler (protected)