fix: log thumbnail fetch failures and restore get() for duplicate-ID safety

Two small correctness fixes from post-commit code review:

- except Exception: continue swallowed network/auth errors silently; add
  logger.debug so systematic failures are diagnosable in winnow.log
- batch_images.pop() regressed duplicate-asset-ID handling: if Immich
  returns the same asset ID twice within the same 32-item batch window
  (pagination edge case), the second occurrence got None and its embedding
  was silently dropped. Switching back to .get() matches the old
  thumbnail_map.get() behaviour. Peak memory is still bounded to _BATCH
  images because batch_images goes out of scope between batches.

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

winnow/diversity.py

@@ -227,12 +227,14 @@ def _select_by_embedding(
                     img = future.result()
                     if img is not None:
                         batch_images[asset["id"]] = img
-                except Exception:
+                except Exception as e:
+                    logger.debug(f"Failed to fetch thumbnail for {asset['id']}: {e}")
                     continue
 
-        # Process and immediately release each image to cap peak memory
+        # Process each image; batch_images goes out of scope after this loop,
+        # bounding peak thumbnail memory to _BATCH images per iteration.
         for asset in batch:
-            img = batch_images.pop(asset["id"], None)
+            img = batch_images.get(asset["id"])
             processed += 1
             if progress_callback:
                 progress_callback(processed, len(candidates))