feat: add traceback logging and spec-aligned error message to GeminiEngine (refs #142)

Co-Authored-By: Claude Opus 4.6 <[email protected]>

ocr/app/engines/gemini_engine.py

@@ -148,13 +148,15 @@ class GeminiEngine:
             return self._model
 
         except ImportError as exc:
+            logger.exception("Vertex AI SDK import failed")
             raise GeminiUnavailableError(
                 "google-cloud-aiplatform is not installed. "
                 "Install with: pip install google-cloud-aiplatform"
             ) from exc
         except Exception as exc:
+            logger.exception("Vertex AI authentication failed")
             raise GeminiUnavailableError(
-                f"Failed to initialize Gemini engine: {exc}"
+                f"Vertex AI authentication failed: {exc}"
             ) from exc
 
     def extract_maintenance(