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/core/store.py

@@ -67,6 +67,8 @@ CREATE TABLE IF NOT EXISTS writeback_log (
 CREATE INDEX IF NOT EXISTS idx_assets_taken_at ON assets(taken_at);
 CREATE INDEX IF NOT EXISTS idx_members_cluster ON cluster_members(cluster_id);
 CREATE INDEX IF NOT EXISTS idx_members_asset ON cluster_members(immich_id);
+CREATE INDEX IF NOT EXISTS idx_writeback_lookup
+  ON writeback_log(immich_id, action, tag, result);
 """
 
 
@@ -109,6 +111,12 @@ class Store:
             self._conn.close()
             self._conn = None
 
+    def __enter__(self) -> "Store":
+        return self
+
+    def __exit__(self, *exc) -> None:
+        self.close()
+
     def upsert_asset(self, a: Asset) -> None:
         has_gps = 1 if (a.gps_lat is not None and a.gps_lon is not None) else 0
         self.conn.execute(
@@ -284,6 +292,10 @@ class Store:
         if boundary_immich_id not in ids:
             raise ValueError(f"boundary {boundary_immich_id!r} not in cluster")
         idx = ids.index(boundary_immich_id)
+        if idx == 0:
+            raise ValueError(
+                f"boundary {boundary_immich_id!r} is the first member; "
+                "split would leave an empty cluster")
         base = self.get_cluster(cluster_id)
         left, right = pairs[:idx], pairs[idx:]
 

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 %}