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/static/app.js

@@ -136,6 +136,8 @@ function clusterReview() {
       location.reload();
     },
     async apply() {
+      // Write-back needs explicit confirmation (mirrors applyAll/merge).
+      if (!confirm('Apply this cluster’s decision to Immich?')) return;
       const r = await this.post(`/cluster/${this.selected}/apply`, {});
       alert(`Applied ${r.succeeded.length}, failed ${r.failed.length}.`);
     },