feat: board-first orchestration with Gateway Bridge, live-update, and flow-board

- GatewayBridgeController: MCP-artiger Kommando-Adapter für Agent-zu-Backend
- TaskBridgeService + LiveUpdateService: SSE Live-Sync + Bridge-Kommandos
- FlowBoard.vue: Board-first orchestration dashboard panel
- live-sync.ts store + live.ts service: SSE-basierte Live-Updates
- Nullability-Warnung in HealthController.cs gefixt
- nginx.conf: SSE-Proxy + CORS für Bridge-Endpunkte
- .gitignore: pnpm/corepack local caches ausgeschlossen
- docs: architecture-board-first-orchestration.md hinzugefügt
- README: Backend Bridge API dokumentiert

backend/Services/ILiveUpdateService.cs

@@ -0,0 +1,17 @@
+using System.Threading.Channels;
+using Nexus.Api.Models;
+
+namespace Nexus.Api.Services;
+
+public interface ILiveUpdateService
+{
+    Task<LiveUpdateSubscription> SubscribeAsync(long? afterSequence = null, CancellationToken ct = default);
+    LiveUpdateEnvelope Publish(string type, object payload, string channel = "dashboard");
+    long CurrentSequence { get; }
+}
+
+public sealed class LiveUpdateSubscription
+{
+    public ChannelReader<LiveUpdateEnvelope> Reader { get; init; } = default!;
+    public long StartingSequence { get; init; }
+}