fix(admin): make force-reanalyze audit transactional; document storage exception

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/api/admin/storage.rs

@@ -205,6 +205,13 @@ async fn backfill(
         resp.more_remaining = more_pages || more_covers;
     }
 
+    // Audit is written after the action here *by necessity*, not oversight
+    // (unlike reenqueue/analyze_page, which wrap their single DB mutation +
+    // audit in one tx). The backfill is a budgeted scan that interleaves
+    // filesystem `storage.size()` reads with per-batch DB writes across many
+    // iterations; wrapping it in a transaction would hold one open across all
+    // that I/O (a long-running tx). The batch size-writes are also idempotent
+    // recomputations, so a lost audit row has negligible impact.
     repo::admin_audit::insert(
         &state.db,
         admin.0.id,