fix(alarm-historian): resolve Medium code-review finding (Core.AlarmHistorian-003)

EnqueueAsync used synchronous SQLite I/O (conn.Open / ExecuteNonQuery /
COUNT(*)) on the caller's thread, blocking the alarm-emitting thread under
write contention with the drain worker. The cancellationToken parameter was
silently ignored.

- EnqueueAsync converted to genuine async: OpenAsync / ExecuteNonQueryAsync /
  ExecuteScalarAsync used throughout; ct threaded to every await.
- ApplyPragmasAsync added alongside the existing ApplyPragmas helper so
  the WAL + busy_timeout PRAGMAs are applied on the async open path too.
- EnforceCapacityAsync added to handle capacity eviction on the async path.

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

code-reviews/Core.AlarmHistorian/findings.md

@@ -63,13 +63,13 @@
 | Severity | Medium |
 | Category | OtOpcUa conventions |
 | Location | `src/Core/ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian/SqliteStoreAndForwardSink.cs:107-127,218-243,246-253` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `EnqueueAsync` is declared `async`-shaped (`Task EnqueueAsync(...)`) and the `IAlarmHistorianSink` contract explicitly states "the sink MUST NOT block the emitting thread … `EnqueueAsync` returns as soon as the queue row is committed." But the implementation does fully synchronous, blocking SQLite I/O (`conn.Open()`, `EnforceCapacity`, `cmd.ExecuteNonQuery()`) on the caller's thread and only then returns `Task.CompletedTask`. Under SQLite write contention with the drain worker this blocks the alarm-emitting thread for the full lock-wait. The same synchronous-work-behind-an-async-or-status-API pattern applies to `GetStatus` (called from the Admin UI / `/healthz` request thread) and `RetryDeadLettered`. The `cancellationToken` parameter of `EnqueueAsync` is accepted and ignored.
 
 **Recommendation:** Either make the I/O genuinely asynchronous (`await conn.OpenAsync(ct)`, `await cmd.ExecuteNonQueryAsync(ct)` — `Microsoft.Data.Sqlite` supports the async surface), or change `EnqueueAsync` to an in-memory hand-off (e.g. a `Channel`) drained by a background writer so the emitting thread truly never touches the database. At minimum honor the `cancellationToken` parameter.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — `EnqueueAsync` now uses `OpenAsync` / `ExecuteNonQueryAsync` / `ExecuteScalarAsync` throughout (capacity check included); `ApplyPragmasAsync` handles the WAL/busy-timeout PRAGMA on the async path; `cancellationToken` is threaded through every await so cancellation is honoured.
 
 ### Core.AlarmHistorian-004