Add JobStateManager for thread-safe job state tracking

Introduced JobStateManager class to manage job states using a ConcurrentDictionary, supporting thread-safe get/set operations per job type. Added State enum with Running and Stopped values. Allows optional initial state dictionary for job states.

EnvelopeGenerator.ServiceHost/Jobs/JobStateManager.cs

@@ -0,0 +1,19 @@
+using AngleSharp.Common;
+using System.Collections.Concurrent;
+
+namespace EnvelopeGenerator.ServiceHost.Jobs;
+
+public class JobStateManager(Dictionary<string, State>? initialState = null)
+{
+    private readonly ConcurrentDictionary<Type, State> _states = new();
+
+    public State GetState<TJob>() => _states.GetOrAdd(typeof(TJob), type => initialState?.GetOrDefault(type.Name, State.Stopped) ?? State.Stopped);
+
+    public State SetState<TJob>(State state) => _states[typeof(TJob)] = state;
+}
+
+public enum State
+{
+    Running,
+    Stopped
+}