feat(55): stop leaving disk debts — temp clips and a low-disk guard

595 green, 0 warnings. Completes TASK 55 (slice 1, log rotation, shipped 2026-10-22).

- **`AlertClipCleanup`**: the default alert clip is materialised into `%TEMP%` for ffmpeg to read, and
  nothing ever deleted it — one orphan per profile that ever played an alert. Now: delete this session's
  file in `Shutdown`, and sweep `ytLive-alert-*.mp4` at startup for anything a crash left behind. Only
  the prefix we own is touched; a directory is never deleted.
- **`DiskSpaceGuard`**: a recording that dies of a full disk is a trust breach, so the numbers are a
  pure policy — refuse to start under **1 GB** (toast; the stream still starts, recording does not),
  warn under **5 GB** at start ("about eighty minutes" at 8 Mbps ≈ 3.6 GB/h), and one mid-show warning
  under **2 GB** — the last useful moment before the encoder fails for a reason it won't explain.
  Unreadable drives (a USB stick that just left) return null and never block a running recording.

Tests are policy-level on purpose: filling a real drive is not a test. `DiskHygieneTests` proves the
sweep only removes our own files and that the thresholds behave at both ends.

Services/AlertClipCleanup.cs

@@ -0,0 +1,60 @@
+using System;
+using System.IO;
+
+namespace ytLive.Services;
+
+/// <summary>
+/// TASK 55·2: the default alert clip is materialised into <c>%TEMP%</c> so ffmpeg can read it as a file.
+/// Nothing used to delete it, so every profile that ever played an alert left an orphan behind.
+///
+/// <para>Two touches, deliberately simple: delete this session's file on close, and sweep anything the
+/// prefix matches at startup (a crash leaves the same debris a clean exit does not). Only files this app
+/// created are touched — the prefix is the contract, and the sweep never deletes a directory.</para>
+/// </summary>
+public static class AlertClipCleanup
+{
+    public const string Prefix = "ytLive-alert-";
+
+    /// <summary>Deletes stale alert clips left in <paramref name="directory"/>. Returns how many went.
+    /// Missing directory is not an error.</summary>
+    public static int Sweep(string directory)
+    {
+        try
+        {
+            if (!Directory.Exists(directory)) return 0;
+            var removed = 0;
+            foreach (var path in Directory.EnumerateFiles(directory, Prefix + "*.mp4"))
+            {
+                if (TryDelete(path)) removed++;
+            }
+            return removed;
+        }
+        catch (Exception ex)
+        {
+            Helpers.AppLog.Write(ex, "Alert clip sweep failed");
+            return 0;
+        }
+    }
+
+    /// <summary>Deletes one clip if it exists. Safe on null/blank and on a locked file.</summary>
+    public static bool Delete(string? path)
+    {
+        if (string.IsNullOrWhiteSpace(path)) return false;
+        return TryDelete(path);
+    }
+
+    private static bool TryDelete(string path)
+    {
+        try
+        {
+            if (!File.Exists(path)) return false;
+            File.Delete(path);
+            return true;
+        }
+        catch (Exception ex)
+        {
+            Helpers.AppLog.Write(ex, $"Alert clip delete failed: {path}");
+            return false;
+        }
+    }
+}