fix(invisible-engine): log exc_info + exception class on restore_faces failure

The InstantID cert sweep emitted `restore_faces post-pass failed ()` -- the
exception's str() was empty so the log line told us nothing about what
actually failed. Adding `exc_info=True` plus `type(e).__name__` so the
full traceback and exception class land in the log even when the message
is empty.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/remove_ai_watermarks/invisible_engine.py

@@ -406,7 +406,12 @@ class InvisibleEngine:
             restored = instantid_restore.restore_faces_instantid(original_bgr, cleaned_bgr, seed=seed)
             image_io.imwrite(out_path, restored)
         except Exception as e:
-            logger.warning("restore_faces post-pass failed (%s); keeping un-restored output", e)
+            logger.warning(
+                "restore_faces post-pass failed (%s: %s); keeping un-restored output",
+                type(e).__name__,
+                e,
+                exc_info=True,
+            )
 
     def _restore_faces_photomaker(
         self,