chore: seal Design Vision v1 and monorepo scaffold

Archive the differentiated YuXinGu product docs, AI engineering system,
design contract, and Go/Vue scaffold. Next execution prioritizes Cece-parity
over early innovation (see .ai/product/STRATEGY.md).

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/internal/handler/health.go

@@ -0,0 +1,25 @@
+package handler
+
+import (
+	"github.com/gin-gonic/gin"
+
+	"github.com/yuxingu/digital-psychology/apps/api/pkg/response"
+)
+
+// HealthHandler serves liveness probes.
+type HealthHandler struct{}
+
+// NewHealthHandler constructs HealthHandler.
+func NewHealthHandler() *HealthHandler {
+	return &HealthHandler{}
+}
+
+// Register mounts health routes on the engine or router group.
+func (h *HealthHandler) Register(r gin.IRoutes) {
+	r.GET("/healthz", h.Healthz)
+}
+
+// Healthz returns a simple OK payload for load balancers.
+func (h *HealthHandler) Healthz(c *gin.Context) {
+	response.OK(c, gin.H{"status": "ok"})
+}