Initial commit: party-mix-app with prefetch cache, audio preload optimizations

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/backend/go.mod

@@ -0,0 +1,14 @@
+module github.com/toyffee/party-mix
+
+go 1.23
+
+require (
+	github.com/gin-contrib/cors v1.7.2
+	github.com/gin-gonic/gin v1.10.0
+	github.com/golang-jwt/jwt/v5 v5.2.1
+	github.com/google/uuid v1.6.0
+	github.com/joho/godotenv v1.5.1
+	golang.org/x/crypto v0.37.0
+	gorm.io/driver/postgres v1.5.9
+	gorm.io/gorm v1.25.12
+)