feat: re-add POST /secret write support (Login working)

Restore CreateCipher/CreateSecret with fixed syntax (quoted map keys,
pkcs7Unpad restored). Login write verified end-to-end.

cmd/api/main.go

@@ -122,6 +122,7 @@ func main() {
 	api.Use(auth.Middleware(auth.NewStore(cfg.APIKeys)))
 
 	api.Get("/secret/:name", h.GetSecret)
+	api.Post("/secret", h.CreateSecret)
 	api.Post("/refresh", h.RefreshCache)
 
 	// Graceful shutdown.