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/models/__init__.py

@@ -1,8 +1,9 @@
 """SQLModel table models. Importing this package registers all tables."""
+from models.agent import Agent
 from models.audit import AuditLog
 from models.setting import Setting, Webhook
 from models.stack import Stack
 from models.template import Template
 from models.user import User
 
-__all__ = ["User", "Stack", "AuditLog", "Template", "Setting", "Webhook"]
+__all__ = ["User", "Stack", "AuditLog", "Template", "Setting", "Webhook", "Agent"]