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/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[:])
+}