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_get_swarm_health.py

@@ -0,0 +1,11 @@
+"""GET /swarm/health — controller liveness (no I/O)."""
+from __future__ import annotations
+
+from fastapi import APIRouter
+
+router = APIRouter()
+
+
+@router.get("/health", tags=["Swarm Health"])
+async def api_get_swarm_health() -> dict[str, str]:
+    return {"status": "ok", "role": "swarm-controller"}