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/Models/Dashboard.cs

@@ -93,7 +93,11 @@ public sealed record DashboardTaskDto(
     bool IsAgentTask = false,
     string? ExpectedFrom = null,
     string? LastActivityMessage = null,
-    DateTimeOffset? LastActivityAt = null
+    DateTimeOffset? LastActivityAt = null,
+    List<DashboardTaskDto>? ChildTasks = null,
+    int ChildTaskCount = 0,
+    int OpenChildTaskCount = 0,
+    bool HasVisibleDelegation = false
 );
 
 public sealed record CreateDashboardTaskRequest(
@@ -182,3 +186,34 @@ public sealed record NotificationDto(
 );
 
 public sealed record UnreadCountDto(int Count);
+
+public sealed record LiveUpdateEnvelope(
+    string Type,
+    DateTimeOffset Timestamp,
+    object Payload,
+    long Sequence,
+    string Channel
+);
+
+public sealed record LiveCursorDto(
+    long Sequence,
+    DateTimeOffset Timestamp,
+    string Mode
+);
+
+public sealed record NotificationSnapshotDto(
+    List<NotificationDto> Notifications,
+    int UnreadCount,
+    string ForUser
+);
+
+public sealed record DashboardLiveSnapshotDto(
+    BoardResponse Board,
+    NotificationSnapshotDto Notifications,
+    LiveCursorDto Cursor
+);
+
+public sealed record DashboardLiveEventDto(
+    LiveUpdateEnvelope Envelope,
+    LiveCursorDto Cursor
+);