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

internal/api/middleware_hosttoken.go

@@ -0,0 +1,30 @@
+package api
+
+import (
+	"net/http"
+
+	"git.omukk.dev/wrenn/sandbox/internal/auth"
+)
+
+// requireHostToken validates the X-Host-Token header containing a host JWT,
+// verifies the signature and expiry, and stamps HostContext into the request context.
+func requireHostToken(secret []byte) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			tokenStr := r.Header.Get("X-Host-Token")
+			if tokenStr == "" {
+				writeError(w, http.StatusUnauthorized, "unauthorized", "X-Host-Token header required")
+				return
+			}
+
+			claims, err := auth.VerifyHostJWT(secret, tokenStr)
+			if err != nil {
+				writeError(w, http.StatusUnauthorized, "unauthorized", "invalid or expired host token")
+				return
+			}
+
+			ctx := auth.WithHostContext(r.Context(), auth.HostContext{HostID: claims.HostID})
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}