feat(backend): rouage gin /api/v1, enveloppe {data,meta}/{error}, health réel, stubs 501 + test de contrat routes

backend/handlers/health.go

@@ -0,0 +1,10 @@
+package handlers
+
+import (
+	"github.com/gin-gonic/gin"
+	"github.com/vaultdrop/backend/pkg/api"
+)
+
+func Health(c *gin.Context) {
+	api.OK(c, gin.H{"status": "healthy"})
+}