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

.gitignore

@@ -34,7 +34,9 @@ docker-compose.override.yml
 **/core
 **/core.*
 
-# pnpm (lockfile IS committed for reproducible CI builds)
+# pnpm / corepack local caches (lockfile IS committed for reproducible CI builds)
+frontend/.pnpm-home/
+frontend/.corepack-home/
 
 # Claude local config (per-developer, not repo-shared)
 .claude/