feat: kontinuierliche LXC↔Mac-Diagnose + Auto-Retry

Diagnose-Sichtbarkeit (Plan A):
- Job.step_started_at + last_heartbeat_at, Heartbeat-Task pingt alle 3 s
  während laufender Mac-Calls
- Mac-Worker hält per X-Job-Id Header einen Log-Ringbuffer pro Job
  (200 Zeilen, 1 h TTL); GET /api/jobs/{id}/log liefert ihn aus
- LXC-Endpoint GET /api/jobs/{id}/diag bündelt Job-Status, Stage-Sekunden,
  Heartbeat-Alter und Mac-Worker-Log
- Frontend: Live-Timing im Status-Pill ("läuft seit Xs · letzter Ping vor Ys"),
  klappbares Diagnose-Panel mit Mac-Log für FAILED + langlaufende Jobs

Robustheit (Plan B):
- MacClient: einmaliger Retry mit 3 s Backoff bei RemoteProtocolError /
  ConnectError / ReadError für summarize/protocol/export
- Mac-Worker /api/preload heizt Ollama vor (keep_alive 30 m, num_predict 1);
  Pipeline ruft Preload vor summarize, damit Modell-Reload nicht mitten
  in der Generierung passiert (Hauptursache "Server disconnected")

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lxc-frontend/app/api/jobs.py

@@ -120,6 +120,8 @@ async def retry_job(
     job.status = JobStatus.QUEUED
     job.progress = 0
     job.error = ""
+    job.step_started_at = None
+    job.last_heartbeat_at = None
     session.add(job)
     session.commit()
     session.refresh(job)
@@ -128,6 +130,21 @@ async def retry_job(
     return _job_dict(job, user)
 
 
+@router.get("/{job_id}/diag")
+async def diag(job_id: int, session: Session = Depends(get_session), user: User = Depends(current_user)):
+    """Diagnose-Bündel: Job-Status + letzte Mac-Worker-Logzeilen für genau diesen Job."""
+    from app.core.mac_client import MacClient
+    job = _require_access(session.get(Job, job_id), user)
+    client = MacClient()
+    mac_log = await client.fetch_log(job_id, last=200)
+    return {
+        "job": _job_dict(job, user),
+        "step_started_at": job.step_started_at.isoformat() if job.step_started_at else None,
+        "last_heartbeat_at": job.last_heartbeat_at.isoformat() if job.last_heartbeat_at else None,
+        "mac_log": mac_log,
+    }
+
+
 @router.get("/{job_id}/download/{kind}")
 def download(job_id: int, kind: str, session: Session = Depends(get_session), user: User = Depends(current_user)):
     job = _require_access(session.get(Job, job_id), user)
@@ -158,6 +175,8 @@ def _job_dict(j: Job, user: User, owner_username: str | None = None) -> dict:
         "profile": j.profile or "meeting",
         "created_at": j.created_at.isoformat(),
         "updated_at": j.updated_at.isoformat(),
+        "step_started_at": j.step_started_at.isoformat() if j.step_started_at else None,
+        "last_heartbeat_at": j.last_heartbeat_at.isoformat() if j.last_heartbeat_at else None,
         "has": {
             "transcript": bool(j.transcript_path),
             "summary": bool(j.summary_path),