Phase 5: multi-host agents (0.5.0)

- stackpilot-agent: slim token-guarded FastAPI (reuses compose_service) exposing
  stack CRUD/lifecycle/logs + system info; same image, different CMD. agent/
  Dockerfile + compose + .env.example.
- Central proxy: Agent model, agent_service (httpx ping/proxy + live status:
  online/offline/unauthorized + hostname/last_seen), routers/agents.py
  (CRUD + ping + proxied stacks/lifecycle/logs/system).
- Frontend: Settings → Remote hosts (add/check/remove, connectivity dot); Stacks
  grouped by host; remote stack detail with lifecycle, live logs, compose/.env edit.

Verified end-to-end: agent+main on a shared network — register (good/bad token),
list/create/start/logs/delete remote stacks, offline detection (502).

Remote backup destinations (SFTP/S3) deferred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -14,6 +14,7 @@ from config import settings
 from database import engine, init_db
 from docker_client import DockerError
 from routers import (
+    agents,
     audit,
     auth,
     backups,
@@ -48,7 +49,7 @@ async def lifespan(app: FastAPI):
     update_task.cancel()
 
 
-app = FastAPI(title="StackPilot", version="0.4.0", lifespan=lifespan)
+app = FastAPI(title="StackPilot", version="0.5.0", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,
@@ -78,6 +79,7 @@ app.include_router(templates.router)
 app.include_router(audit.router)
 app.include_router(settings_router.router)
 app.include_router(backups.router)
+app.include_router(agents.router)
 app.include_router(ws.router)