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

shared/photoflow/ui/templates/macros.html

@@ -10,7 +10,7 @@
   {% elif confidence < 0.75 %}
     <span class="badge badge-sm badge-info">med</span>
   {% else %}
-    <span class="badge badge-sm badge-success">high</span>
+    <span class="badge badge-sm badge-success" title="high-confidence">high</span>
   {% endif %}
 {% endmacro %}