feat: add thread-safe ActivityLog with bounded capacity

src/SpamGuard/Services/ActivityLog.cs

@@ -0,0 +1,48 @@
+namespace SpamGuard.Services;
+
+using SpamGuard.Models;
+
+public sealed class ActivityLog
+{
+    private readonly List<ActivityEntry> _entries = new();
+    private readonly object _lock = new();
+    private readonly int _maxEntries;
+
+    public int TodayChecked => GetTodayCount(_ => true);
+    public int TodaySpam => GetTodayCount(e => e.Verdict == Verdict.Spam);
+
+    public ActivityLog(int maxEntries = 500)
+    {
+        _maxEntries = maxEntries;
+    }
+
+    public void Add(ActivityEntry entry)
+    {
+        lock (_lock)
+        {
+            _entries.Add(entry);
+            if (_entries.Count > _maxEntries)
+                _entries.RemoveAt(0);
+        }
+    }
+
+    public List<ActivityEntry> GetRecent(int count = 100)
+    {
+        lock (_lock)
+        {
+            return _entries
+                .OrderByDescending(e => e.Timestamp)
+                .Take(count)
+                .ToList();
+        }
+    }
+
+    private int GetTodayCount(Func<ActivityEntry, bool> predicate)
+    {
+        var today = DateTime.UtcNow.Date;
+        lock (_lock)
+        {
+            return _entries.Count(e => e.Timestamp.Date == today && predicate(e));
+        }
+    }
+}