feat: add Cache-Control: private, no-store to all API routes per default (#1126)

backend/internal/middleware/cache_control.go

@@ -0,0 +1,26 @@
+package middleware
+
+import "github.com/gin-gonic/gin"
+
+// CacheControlMiddleware sets a safe default Cache-Control header on responses
+// that do not already specify one. This prevents proxies from caching
+// authenticated responses that might contain private data.
+type CacheControlMiddleware struct {
+	headerValue string
+}
+
+func NewCacheControlMiddleware() *CacheControlMiddleware {
+	return &CacheControlMiddleware{
+		headerValue: "private, no-store",
+	}
+}
+
+func (m *CacheControlMiddleware) Add() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if c.Writer.Header().Get("Cache-Control") == "" {
+			c.Header("Cache-Control", m.headerValue)
+		}
+
+		c.Next()
+	}
+}