Watchdog-Heartbeat + LicenseLabrador-Lizenzgate im WinFormsHost

Bindet die beiden neuen Betriebsprojekte an:

- WatchdogHeartbeatService: periodischer POST /api/heartbeat an
  watchdog.mhdf.de (Dead-Man's-Switch), stopping-Event beim Beenden,
  Konfiguration ueber AppSettings-Kategorie "Watchdog". Fehler sind
  best effort und beeintraechtigen die App nie.
- LicenseGuard + LicenseDialog: Lizenzpruefung vor dem Start der
  MainForm, 12h-Revalidierung zur Laufzeit, Checksum-Haertung.
  Produkt-Slug/Endpoint/Public-Key sind einkompiliert.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Predictalytics.WinFormsHost/Services/LicenseDialog.cs

@@ -0,0 +1,118 @@
+using LicenseLabrador.Client;
+
+namespace Predictalytics.WinFormsHost.Services;
+
+/// <summary>
+/// Modal dialog shown at startup when no usable license is present.
+/// Lets the user enter/activate a license key; closes with OK only after a
+/// successful, checksum-verified validation.
+/// </summary>
+public sealed class LicenseDialog : Form
+{
+    private readonly LicenseClient _client;
+    private readonly Label _lblStatus;
+    private readonly TextBox _txtKey;
+    private readonly Button _btnActivate;
+    private readonly Button _btnExit;
+
+    public LicenseResult? Result { get; private set; }
+
+    public LicenseDialog(LicenseClient client, LicenseResult? lastResult)
+    {
+        _client = client;
+
+        Text = "Predictalytics — Lizenzaktivierung";
+        FormBorderStyle = FormBorderStyle.FixedDialog;
+        MaximizeBox = false;
+        MinimizeBox = false;
+        StartPosition = FormStartPosition.CenterScreen;
+        ClientSize = new Size(460, 190);
+
+        var lblInfo = new Label
+        {
+            Text = "Diese Installation benötigt eine gültige Lizenz.\nBitte Lizenzschlüssel eingeben (Format: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX):",
+            Location = new Point(12, 12),
+            Size = new Size(436, 34)
+        };
+
+        _txtKey = new TextBox
+        {
+            Location = new Point(12, 52),
+            Size = new Size(436, 26),
+            Font = new Font("Consolas", 11f),
+            CharacterCasing = CharacterCasing.Upper
+        };
+
+        _lblStatus = new Label
+        {
+            Location = new Point(12, 84),
+            Size = new Size(436, 50),
+            ForeColor = Color.Firebrick,
+            Text = FormatInitialStatus(lastResult)
+        };
+
+        _btnActivate = new Button
+        {
+            Text = "Aktivieren",
+            Location = new Point(252, 146),
+            Size = new Size(96, 30)
+        };
+        _btnActivate.Click += async (_, _) => await ActivateAsync();
+
+        _btnExit = new Button
+        {
+            Text = "Beenden",
+            Location = new Point(354, 146),
+            Size = new Size(94, 30),
+            DialogResult = DialogResult.Cancel
+        };
+
+        AcceptButton = _btnActivate;
+        CancelButton = _btnExit;
+        Controls.AddRange(new Control[] { lblInfo, _txtKey, _lblStatus, _btnActivate, _btnExit });
+    }
+
+    private static string FormatInitialStatus(LicenseResult? lastResult)
+    {
+        if (lastResult == null || lastResult.State == LicenseState.NoLicense) return "";
+        return $"Letzte Prüfung: {lastResult.State} — {lastResult.Message}";
+    }
+
+    private async Task ActivateAsync()
+    {
+        var key = _txtKey.Text.Trim();
+        if (string.IsNullOrWhiteSpace(key))
+        {
+            _lblStatus.Text = "Bitte einen Lizenzschlüssel eingeben.";
+            return;
+        }
+
+        _btnActivate.Enabled = false;
+        _lblStatus.ForeColor = Color.DimGray;
+        _lblStatus.Text = "Prüfe Lizenz am Server...";
+
+        try
+        {
+            var result = await _client.ValidateAsync(key);
+            if (result.IsUsable && _client.VerifyChecksum(result))
+            {
+                Result = result;
+                DialogResult = DialogResult.OK;
+                Close();
+                return;
+            }
+
+            _lblStatus.ForeColor = Color.Firebrick;
+            _lblStatus.Text = $"Lizenz nicht nutzbar ({result.State}):\n{result.Message}";
+        }
+        catch (Exception ex)
+        {
+            _lblStatus.ForeColor = Color.Firebrick;
+            _lblStatus.Text = $"Fehler bei der Prüfung: {ex.Message}";
+        }
+        finally
+        {
+            _btnActivate.Enabled = true;
+        }
+    }
+}