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/config/config.go

@@ -0,0 +1,27 @@
+// Package config loads process configuration from environment variables.
+package config
+
+import "os"
+
+// Config holds runtime settings for the API server.
+type Config struct {
+	HTTPAddr    string
+	DatabaseURL string
+	AppEnv      string
+}
+
+// Load reads configuration from the environment with safe defaults for local dev.
+func Load() Config {
+	return Config{
+		HTTPAddr:    getenv("HTTP_ADDR", ":8080"),
+		DatabaseURL: getenv("DATABASE_URL", "postgres://yuxingu:yuxingu@127.0.0.1:5432/yuxingu?sslmode=disable"),
+		AppEnv:      getenv("APP_ENV", "dev"),
+	}
+}
+
+func getenv(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}

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"})
+}

apps/api/internal/middleware/request_id.go

@@ -0,0 +1,29 @@
+package middleware
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+
+	"github.com/gin-gonic/gin"
+)
+
+// RequestID attaches X-Request-ID to every request/response for tracing.
+func RequestID() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		id := c.GetHeader("X-Request-ID")
+		if id == "" {
+			id = newID()
+		}
+		c.Set("request_id", id)
+		c.Header("X-Request-ID", id)
+		c.Next()
+	}
+}
+
+func newID() string {
+	var b [16]byte
+	if _, err := rand.Read(b[:]); err != nil {
+		return "req-unknown"
+	}
+	return hex.EncodeToString(b[:])
+}