fix(review): harden write-back, confirmation gate, and edge cases

- write-back: call upsert_tag inside _apply_tag's try so a tag-create failure
  is caught per-asset and apply_all no longer aborts mid-batch (was P1)
- write-back: surface _pipeline/processed write failures in the result instead
  of discarding them (was reported as success)
- ui: add title to the high confidence badge so approve-high-confidence's
  pre-action count is non-zero; confirm() before single-cluster apply
- core: Store context manager; close DB connection even if a route raises;
  guard split_cluster against a first-member boundary (empty cluster); add
  writeback_log lookup index
- ingest: split thumbnail download/write error handling and clean up the
  .tmp file on a write failure
- tests: upsert/processed write-failure regression tests + split-guard test

apps/trip-cluster/app/ingest.py

@@ -45,14 +45,23 @@ def run_ingest(client, store, thumbs_dir, *, date_from=None, date_to=None,
         if need_thumb:
             try:
                 data = client.download_thumbnail(a["id"])
-                # Write to a temp path and os.replace into place so a failure
-                # never leaves a 0-byte <id>.jpg behind.
-                tmp_path = f"{thumb_path}.tmp"
+            except Exception:
+                log.exception("Thumbnail download failed for asset %s", a["id"])
+                continue
+            # Write to a temp path and os.replace into place so a failure
+            # never leaves a 0-byte <id>.jpg behind; clean up the temp file
+            # if the write itself fails (e.g. disk full).
+            tmp_path = f"{thumb_path}.tmp"
+            try:
                 with open(tmp_path, "wb") as f:
                     f.write(data)
                 os.replace(tmp_path, thumb_path)
             except Exception:
-                log.exception("Thumbnail download failed for asset %s", a["id"])
+                log.exception("Thumbnail write failed for asset %s", a["id"])
+                try:
+                    os.unlink(tmp_path)
+                except OSError:
+                    pass
                 continue
 
         store.upsert_asset(Asset(