fix: heartbeat 500 error, timezone to Beijing, tasks guide collapse

- fix(api): heartbeat endpoint no longer requires request body, fix spa_fallback
  404 handler causing 500 on API routes, add db.refresh() in agent update/heartbeat
- feat(time): unify all frontend time display to Beijing time (Asia/Shanghai)
  via new beijing() utility; backend alert messages also use BJT
- feat(ui): collapse AI agent instruction panel on Tasks page by default
- chore: add backend/app/utils.py and frontend/src/utils/time.js

backend/app/api/agents.py

@@ -50,7 +50,7 @@ async def update_agent(agent_id: int, body: AgentUpdate, db: AsyncSession = Depe
 
 
 @router.post("/{agent_id}/heartbeat", response_model=AgentOut)
-async def heartbeat(agent_id: int, body: AgentHeartbeat, db: AsyncSession = Depends(get_db)):
+async def heartbeat(agent_id: int, db: AsyncSession = Depends(get_db)):
     svc = AgentService(db)
     agent = await svc.heartbeat(agent_id)
     if not agent:

backend/app/main.py

@@ -74,7 +74,8 @@ if FRONTEND_DIST.is_dir():
         if not request.url.path.startswith("/api"):
             content = (FRONTEND_DIST / "index.html").read_text(encoding="utf-8")
             return HTMLResponse(content=content, status_code=200)
-        raise exc
+        from fastapi.responses import JSONResponse
+        return JSONResponse(status_code=404, content={"detail": "Not Found"})
 
     logger.info("Frontend SPA mounted from %s", FRONTEND_DIST)
 else:

backend/app/services/agent.py

@@ -38,6 +38,7 @@ class AgentService:
             if v is not None and hasattr(agent, k):
                 setattr(agent, k, v)
         await self.db.flush()
+        await self.db.refresh(agent)
         return agent
 
     async def heartbeat(self, agent_id: int) -> Agent | None:
@@ -46,6 +47,7 @@ class AgentService:
         if agent:
             agent.last_heartbeat_at = datetime.utcnow()
             await self.db.flush()
+            await self.db.refresh(agent)
         return agent
 
     async def get_task_count(self, agent_id: int) -> int:

backend/app/services/scheduler.py

@@ -10,6 +10,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.config import settings
 from app.database import async_session_factory
 from app.models import ScheduledTask, Alert, AlertRule, NotificationChannel
+from app.utils import fmt_bj
 
 logger = logging.getLogger("taskpulse.scheduler")
 
@@ -66,9 +67,9 @@ class SchedulerService:
                     alert_type="missed_run",
                     message=(
                         f"任务 [{task.name}](ID={task.id}) 超过预定时间未执行。\n"
-                        f"预期执行时间: {task.next_run_at}\n"
+                        f"预期执行时间: {fmt_bj(task.next_run_at)}\n"
                         f"容忍窗口: {task.grace_period}s\n"
-                        f"当前时间: {now}"
+                        f"当前时间: {fmt_bj(now)}"
                     ),
                     status="pending",
                 )

backend/app/utils.py

@@ -0,0 +1,29 @@
+"""Time utility — Beijing time (Asia/Shanghai, UTC+8)."""
+
+from datetime import datetime, timezone, timedelta, tzinfo
+
+# Beijing timezone (UTC+8, no DST)
+_BJ_TZ = timezone(timedelta(hours=8))
+
+
+def now() -> datetime:
+    """Return current time in UTC (naive, for DB storage)."""
+    return datetime.utcnow()
+
+
+def now_bj() -> datetime:
+    """Return current time as Beijing time (aware)."""
+    return datetime.now(_BJ_TZ)
+
+
+def fmt_bj(dt: datetime | None) -> str:
+    """Format a naive datetime (assumed UTC) as Beijing time string.
+    
+    Returns e.g. '2026-06-15 10:30:00'
+    """
+    if dt is None:
+        return "—"
+    # Assume naive dt is UTC
+    utc_dt = dt.replace(tzinfo=timezone.utc)
+    bj_dt = utc_dt.astimezone(_BJ_TZ)
+    return bj_dt.strftime("%Y-%m-%d %H:%M:%S")