feat(server): implement sealed share lifecycle

Signed-off-by: JustMe <mr.toing@outlook.com>

server/src/WebDrop.Api/SweepWorker.cs

@@ -0,0 +1,42 @@
+using WebDrop.Core;
+
+namespace WebDrop.Api;
+
+/// <summary>定期清理过期条目、下载达上限的条目、过期频道,并低频清理孤立 blob。</summary>
+public sealed class SweepWorker(ChannelService service, ShareService shares, WebDropOptions options, ILogger<SweepWorker> logger) : BackgroundService
+{
+    private const int OrphanSweepEvery = 20;
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        var interval = TimeSpan.FromSeconds(Math.Max(1, options.SweepIntervalSeconds));
+        using var timer = new PeriodicTimer(interval);
+        int round = 0;
+
+        do
+        {
+            try
+            {
+                bool orphans = round++ % OrphanSweepEvery == 0;
+                int removed = await Task.Run(() => service.Sweep(orphans) + shares.Sweep(), stoppingToken).ConfigureAwait(false);
+                if (removed > 0) logger.LogInformation("清理了 {Count} 个过期条目或频道", removed);
+            }
+            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
+            {
+                break;
+            }
+            catch (Exception e)
+            {
+                // 只记录异常类型,不记录可能含频道 id 的细节
+                logger.LogError("清理任务失败:{Type}", e.GetType().Name);
+            }
+        }
+        while (await WaitAsync(timer, stoppingToken).ConfigureAwait(false));
+    }
+
+    private static async Task<bool> WaitAsync(PeriodicTimer timer, CancellationToken ct)
+    {
+        try { return await timer.WaitForNextTickAsync(ct).ConfigureAwait(false); }
+        catch (OperationCanceledException) { return false; }
+    }
+}