feat(agent): SSE — progressive Themen-Tabs (Phase 2)

Der Compliance-Check streamt jetzt progressive Events; der Impressum-Tab
erscheint, sobald das Thema fertig ist, statt am Ende alles auf einmal.
Additiv — das Polling fürs finale Ergebnis bleibt.

- backend: _sse.py (Queue/emit/event_generator) + Endpoint
  /compliance-check/{id}/stream; _update emittiert progress,
  run_agent_outputs emittiert topic (laeuft jetzt frueh nach Phase B),
  Orchestrator emittiert complete/error.
- frontend: SSE-Proxy-Route + EventSource in ComplianceCheckTab merged
  topic-Events in agent_outputs -> Tab erscheint progressiv.
- Tests: backend 5 passed (SSE + agent_outputs); tsc 0 neue Fehler,
  vitest 2 passed, check-loc 0.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

admin-compliance/app/api/sdk/v1/agent/compliance-check/[check_id]/stream/route.ts

@@ -0,0 +1,41 @@
+/**
+ * Compliance-Check SSE-Proxy
+ * GET /api/sdk/v1/agent/compliance-check/{check_id}/stream
+ *   → backend /api/compliance/agent/compliance-check/{check_id}/stream
+ *
+ * Reicht den text/event-stream-Body unmodifiziert durch (progressive
+ * topic-/progress-Events fürs Frontend). Additiv zum Polling.
+ */
+
+import { NextRequest, NextResponse } from 'next/server'
+
+const BACKEND_URL =
+  process.env.BACKEND_API_URL || process.env.BACKEND_URL ||
+  'http://backend-compliance:8002'
+
+export async function GET(
+  _request: NextRequest,
+  { params }: { params: Promise<{ check_id: string }> },
+) {
+  const { check_id } = await params
+  try {
+    const response = await fetch(
+      `${BACKEND_URL}/api/compliance/agent/compliance-check/${check_id}/stream`,
+      { signal: AbortSignal.timeout(1_800_000) }, // 30 min
+    )
+    return new NextResponse(response.body, {
+      status: response.status,
+      headers: {
+        'Content-Type': 'text/event-stream',
+        'Cache-Control': 'no-cache',
+        'Connection': 'keep-alive',
+        'X-Accel-Buffering': 'no',
+      },
+    })
+  } catch {
+    return NextResponse.json(
+      { error: 'SSE-Stream zum Backend fehlgeschlagen' },
+      { status: 503 },
+    )
+  }
+}