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/id/id.go

@@ -38,3 +38,22 @@ func NewTeamID() string {
 func NewAPIKeyID() string {
 	return "key-" + hex8()
 }
+
+// NewHostID generates a new host ID in the format "host-" + 8 hex chars.
+func NewHostID() string {
+	return "host-" + hex8()
+}
+
+// NewHostTokenID generates a new host token audit ID in the format "htok-" + 8 hex chars.
+func NewHostTokenID() string {
+	return "htok-" + hex8()
+}
+
+// NewRegistrationToken generates a 64-char hex token (32 bytes of entropy).
+func NewRegistrationToken() string {
+	b := make([]byte, 32)
+	if _, err := rand.Read(b); err != nil {
+		panic(fmt.Sprintf("crypto/rand failed: %v", err))
+	}
+	return hex.EncodeToString(b)
+}