feat: 3 Scan-Modi (Quick/Web/Full) für Key Scanner

- Quick-Scan: nur GitHub mit Dotenv, Text, Markdown + .env
- Web-Scan: nur Serper.dev (Google Dorks)
- Full-Scan: GitHub + Serper komplett (wie bisher)
- Backend: TriggerScan(mode), ScanGitHubQuick(), mode-basierte Quellsteuerung
- Endpoint: ?mode=quick|web|full Query-Parameter
- Frontend: 3 Buttons (⚡ Quick, 🌐 Web, 🔍 Full) mit gemeinsamer Scan-Logik
- Status-Anzeige zeigt Scan-Modus im Ergebnis

Endpoints/ApiKeysEndpoint.cs

@@ -238,8 +238,12 @@ public static class ApiKeysEndpoint
             return ctx.Response.WriteAsJsonAsync(new { error = "Scan läuft bereits" });
         }
 
-        scanner.TriggerScan();
-        return ctx.Response.WriteAsJsonAsync(new { ok = true, message = "Scan gestartet" });
+        var mode = ctx.Request.Query["mode"].FirstOrDefault() ?? "full";
+        var validModes = new[] { "quick", "web", "full" };
+        if (!validModes.Contains(mode)) mode = "full";
+
+        scanner.TriggerScan(mode);
+        return ctx.Response.WriteAsJsonAsync(new { ok = true, message = $"{mode}-Scan gestartet" });
     }
 
     private static Task HandleScanStatus(HttpContext ctx, KeyScannerService scanner)