Add host registration, heartbeat, and multi-host management

Implements the full host ↔ control plane connection flow:

- Host CRUD endpoints (POST/GET/DELETE /v1/hosts) with role-based access:
  regular hosts admin-only, BYOC hosts for admins and team owners
- One-time registration token flow: admin creates host → gets token (1hr TTL
  in Redis + Postgres audit trail) → host agent registers with specs → gets
  long-lived JWT (1yr)
- Host agent registration client with automatic spec detection (arch, CPU,
  memory, disk) and token persistence to disk
- Periodic heartbeat (30s) via POST /v1/hosts/{id}/heartbeat with X-Host-Token
  auth and host ID cross-check
- Token regeneration endpoint (POST /v1/hosts/{id}/token) for retry after
  failed registration
- Tag management (add/remove/list) with team-scoped access control
- Host JWT with typ:"host" claim, cross-use prevention in both VerifyJWT and
  VerifyHostJWT
- requireHostToken middleware for host agent authentication
- DB-level race protection: RegisterHost uses AND status='pending' with
  rows-affected check; Redis GetDel for atomic token consume
- Migration for future mTLS support (cert_fingerprint, mtls_enabled columns)
- Host agent flags: --register (one-time token), --address (required ip:port)
- serviceErrToHTTP extended with "forbidden" → 403 mapping
- OpenAPI spec, .env.example, and README updated

cmd/control-plane/main.go

@@ -66,8 +66,6 @@ func main() {
 	}
 	slog.Info("connected to redis")
 
-	_ = rdb // TODO: pass to services that need it (host registration)
-
 	// Connect RPC client for the host agent.
 	agentHTTP := &http.Client{Timeout: 10 * time.Minute}
 	agentClient := hostagentv1connect.NewHostAgentServiceClient(
@@ -89,7 +87,7 @@ func main() {
 	}
 
 	// API server.
-	srv := api.New(queries, agentClient, pool, []byte(cfg.JWTSecret), oauthRegistry, cfg.OAuthRedirectURL)
+	srv := api.New(queries, agentClient, pool, rdb, []byte(cfg.JWTSecret), oauthRegistry, cfg.OAuthRedirectURL)
 
 	// Start reconciler.
 	reconciler := api.NewReconciler(queries, agentClient, "default", 5*time.Second)