Convert backend from submodule to regular directory

Remove submodule tracking; backend is now a plain directory in the repo.
Also update deploy workflow: remove --recurse-submodules.

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

backend/internal/api/response.go

@@ -0,0 +1,21 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	json.NewEncoder(w).Encode(v)
+}
+
+type errorResponse struct {
+	Code    string `json:"code"`
+	Message string `json:"message"`
+}
+
+func writeError(w http.ResponseWriter, status int, code, message string) {
+	writeJSON(w, status, errorResponse{Code: code, Message: message})
+}