Refactor source directory handling in modelsProxyService.js

- Introduced a cached source directory variable to optimize the retrieval of model paths.
- Updated comments for clarity on the source directory logic and its impact on cache validation.

backend/services/modelsProxyService.js

@@ -17,9 +17,15 @@ const PUBLIC_MODELS = path.join(PROJECT_ROOT, 'frontend', 'public', MODELS_REL);
 const CACHE_DIR = path.join(BACKEND_DIR, 'data', 'model-cache');
 const CLI_PATH = path.join(BACKEND_DIR, 'node_modules', '.bin', 'gltf-transform');
 
-/** Production: frontend/dist; Local: frontend/public. Ermittelt bei Bedarf. */
+/** Einmal ermitteltes Quellverzeichnis (frontend/dist oder frontend/public). */
+let _sourceDir = null;
+
+/** Production: frontend/dist; Local: frontend/public. Einmal pro Prozess festgelegt, damit
+ * isCacheValid() stets gegen dieselbe Quelle prüft (kein Wechsel zwischen dist/public). */
 function getSourceDir() {
-  return fs.existsSync(DIST_MODELS) ? DIST_MODELS : PUBLIC_MODELS;
+  if (_sourceDir !== null) return _sourceDir;
+  _sourceDir = fs.existsSync(DIST_MODELS) ? DIST_MODELS : PUBLIC_MODELS;
+  return _sourceDir;
 }
 
 /** Erlaubte Dateinamen (nur [a-z0-9_.-]+.glb) */