fix(stage-3): queue DL fan-out + emit-failure visibility

Closes the queue DL fan-out gap (audit Medium) and surfaces the
KV/docs/files non-transactional emit gap to operators.

- handle_queue_failure previously called queue.dead_letter to write the
  DL row but never invoked fan_out_dead_letter. The comment claimed
  "the outbox arm fires registered dead_letter handlers off the new row"
  — but queue DL rows are written via a separate path that bypasses the
  outbox entirely, so handlers filtered on source="queue" sat idle
  forever. Refactor fan_out_dead_letter to take a DeadLetterFanOutCtx
  struct so both the outbox arm and the queue arm can call it; the
  queue arm constructs a TriggerEvent::Queue from the claimed message
  and passes it through.

- New integration test queue_dead_letter_fans_out_to_dead_letter_handler
  registers a dead_letter trigger filtered on "queue", forces queue
  exhaustion, asserts the handler fires with the correctly-shaped event.

- KV/docs/files services committed the data write then ran events.emit
  as a separate operation, logging-and-swallowing on failure. Bump the
  six call sites from tracing::warn to tracing::error with an
  event_emit_failure=true marker so operators can grep them. The full
  single-tx repo refactor (extending ServiceEventEmitter with
  emit_in_tx + tx-aware *_repo methods) is documented in kv_service.rs
  as a v1.2 follow-up — it's a meaningful redesign that deserves its
  own pass (pubsub_service::fan_out_publish is the reference shape).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/manager-core/src/docs_service.rs

@@ -184,7 +184,7 @@ impl DocsService for DocsServiceImpl {
             )
             .await
         {
-            tracing::warn!(error = %e, source = "docs", op = "create", "event emit failed");
+            tracing::error!(error = %e, source = "docs", op = "create", event_emit_failure = true, "event emit failed");
         }
         Ok(row.id)
     }
@@ -262,7 +262,7 @@ impl DocsService for DocsServiceImpl {
                     )
                     .await
                 {
-                    tracing::warn!(error = %e, source = "docs", op = "update", "event emit failed");
+                    tracing::error!(error = %e, source = "docs", op = "update", event_emit_failure = true, "event emit failed");
                 }
                 Ok(())
             }
@@ -291,7 +291,7 @@ impl DocsService for DocsServiceImpl {
                 )
                 .await
             {
-                tracing::warn!(error = %e, source = "docs", op = "delete", "event emit failed");
+                tracing::error!(error = %e, source = "docs", op = "delete", event_emit_failure = true, "event emit failed");
             }
         }
         Ok(was_present)