refactor(swarm): one file per endpoint, matching existing router layout

Splits the three grouped router files into eight api_<verb>_<resource>.py
modules under decnet/web/router/swarm/ to match the convention used by
router/fleet/ and router/config/. Shared request/response models live in
_schemas.py. Keeps each endpoint easy to locate and modify without
stepping on siblings.

decnet/web/router/swarm/api_check_hosts.py

@@ -0,0 +1,61 @@
+"""POST /swarm/check — active mTLS probe of every enrolled worker.
+
+Updates ``SwarmHost.status`` and ``last_heartbeat`` for each host based
+on the outcome of the probe.
+"""
+from __future__ import annotations
+
+import asyncio
+from datetime import datetime, timezone
+from typing import Any
+
+from fastapi import APIRouter, Depends
+
+from decnet.logging import get_logger
+from decnet.swarm.client import AgentClient
+from decnet.web.db.repository import BaseRepository
+from decnet.web.dependencies import get_repo
+from decnet.web.router.swarm._schemas import CheckResponse, HostHealth
+
+log = get_logger("swarm.check")
+
+router = APIRouter()
+
+
+@router.post("/check", response_model=CheckResponse, tags=["Swarm Health"])
+async def api_check_hosts(
+    repo: BaseRepository = Depends(get_repo),
+) -> CheckResponse:
+    hosts = await repo.list_swarm_hosts()
+
+    async def _probe(host: dict[str, Any]) -> HostHealth:
+        try:
+            async with AgentClient(host=host) as agent:
+                body = await agent.health()
+            await repo.update_swarm_host(
+                host["uuid"],
+                {
+                    "status": "active",
+                    "last_heartbeat": datetime.now(timezone.utc),
+                },
+            )
+            return HostHealth(
+                host_uuid=host["uuid"],
+                name=host["name"],
+                address=host["address"],
+                reachable=True,
+                detail=body,
+            )
+        except Exception as exc:
+            log.warning("swarm.check unreachable host=%s err=%s", host["name"], exc)
+            await repo.update_swarm_host(host["uuid"], {"status": "unreachable"})
+            return HostHealth(
+                host_uuid=host["uuid"],
+                name=host["name"],
+                address=host["address"],
+                reachable=False,
+                detail=str(exc),
+            )
+
+    results = await asyncio.gather(*(_probe(h) for h in hosts))
+    return CheckResponse(results=list(results))