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"]

backend/models/agent.py

@@ -0,0 +1,49 @@
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from typing import Optional
+
+from sqlmodel import Field, SQLModel
+
+
+def _now() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+class Agent(SQLModel, table=True):
+    """A remote host running stackpilot-agent."""
+
+    id: Optional[int] = Field(default=None, primary_key=True)
+    name: str
+    url: str  # e.g. http://10.0.0.5:5010
+    token: str  # shared AGENT_TOKEN of that host
+    status: str = "unknown"  # online | offline | unauthorized | unknown
+    hostname: Optional[str] = None  # reported by the agent on ping
+    last_seen: Optional[datetime] = None
+    created_at: datetime = Field(default_factory=_now)
+
+
+# --- API schemas ---
+
+
+class AgentCreate(SQLModel):
+    name: str
+    url: str
+    token: str
+
+
+class AgentUpdate(SQLModel):
+    name: Optional[str] = None
+    url: Optional[str] = None
+    token: Optional[str] = None
+
+
+class AgentRead(SQLModel):
+    id: int
+    name: str
+    url: str
+    status: str
+    hostname: Optional[str]
+    last_seen: Optional[datetime]
+    created_at: datetime
+    token_set: bool