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>

backend-compliance/compliance/api/agent_compliance_check_routes.py

@@ -30,6 +30,7 @@ import uuid as _uuid
 
 import httpx
 from fastapi import APIRouter
+from fastapi.responses import StreamingResponse
 
 # ── Re-exports: external callers import these from THIS module ──────
 from .agent_check._constants import (  # noqa: F401
@@ -63,6 +64,7 @@ from .agent_check._schemas import (
     ExtractTextRequest,
 )
 from .agent_check._single_check import _check_single  # noqa: F401
+from .agent_check._sse import event_generator, new_queue
 
 logger = logging.getLogger(__name__)
 
@@ -137,6 +139,7 @@ async def start_compliance_check(req: ComplianceCheckRequest):
         "result": None,
         "error": "",
     }
+    new_queue(check_id)  # SSE: progressive topic-Events fürs Frontend
     asyncio.create_task(_run_compliance_check(check_id, req))
     return ComplianceCheckStartResponse(check_id=check_id, status="running")
 
@@ -157,6 +160,21 @@ async def get_compliance_check_status(check_id: str):
     )
 
 
+@router.get("/compliance-check/{check_id}/stream")
+async def stream_compliance_check(check_id: str) -> StreamingResponse:
+    """SSE-Stream: progressive Events (progress/topic/complete) eines
+    laufenden Checks. Additiv zum Polling auf /compliance-check/{check_id}."""
+    return StreamingResponse(
+        event_generator(check_id),
+        media_type="text/event-stream",
+        headers={
+            "Cache-Control": "no-cache",
+            "X-Accel-Buffering": "no",   # nginx: nicht puffern
+            "Connection": "keep-alive",
+        },
+    )
+
+
 # ── P80: Snapshot + Replay ───────────────────────────────────────────
 
 @router.get("/snapshots")