feat(api): add OpenAPI documentation

backend/internal/httpapi/docs.go

@@ -0,0 +1,53 @@
+package httpapi
+
+import (
+	_ "embed"
+	"net/http"
+)
+
+//go:embed openapi.json
+var openAPIDocument []byte
+
+const swaggerUI = `<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>Card to IBAN API documentation</title>
+  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/swagger-ui-dist@5/swagger-ui.css">
+</head>
+<body>
+  <div id="swagger-ui"></div>
+  <script src="https://cdn.jsdelivr.net/npm/swagger-ui-dist@5/swagger-ui-bundle.js"></script>
+  <script>
+    window.onload = () => SwaggerUIBundle({
+      url: "/openapi.json",
+      dom_id: "#swagger-ui",
+      deepLinking: true,
+      presets: [SwaggerUIBundle.presets.apis]
+    });
+  </script>
+</body>
+</html>
+`
+
+// RegisterDocumentation registers the OpenAPI document and its Swagger UI.
+func RegisterDocumentation(mux *http.ServeMux) {
+	mux.HandleFunc("GET /openapi.json", serveOpenAPI)
+	mux.HandleFunc("GET /swagger/", serveSwaggerUI)
+	mux.HandleFunc("GET /docs", redirectToSwaggerUI)
+}
+
+func serveOpenAPI(w http.ResponseWriter, _ *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	_, _ = w.Write(openAPIDocument)
+}
+
+func serveSwaggerUI(w http.ResponseWriter, _ *http.Request) {
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	_, _ = w.Write([]byte(swaggerUI))
+}
+
+func redirectToSwaggerUI(w http.ResponseWriter, r *http.Request) {
+	http.Redirect(w, r, "/swagger/", http.StatusTemporaryRedirect)
+}