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/db/teams.sql.go

@@ -73,6 +73,28 @@ func (q *Queries) GetTeam(ctx context.Context, id string) (Team, error) {
 	return i, err
 }
 
+const getTeamMembership = `-- name: GetTeamMembership :one
+SELECT user_id, team_id, is_default, role, created_at FROM users_teams WHERE user_id = $1 AND team_id = $2
+`
+
+type GetTeamMembershipParams struct {
+	UserID string `json:"user_id"`
+	TeamID string `json:"team_id"`
+}
+
+func (q *Queries) GetTeamMembership(ctx context.Context, arg GetTeamMembershipParams) (UsersTeam, error) {
+	row := q.db.QueryRow(ctx, getTeamMembership, arg.UserID, arg.TeamID)
+	var i UsersTeam
+	err := row.Scan(
+		&i.UserID,
+		&i.TeamID,
+		&i.IsDefault,
+		&i.Role,
+		&i.CreatedAt,
+	)
+	return i, err
+}
+
 const insertTeam = `-- name: InsertTeam :one
 INSERT INTO teams (id, name)
 VALUES ($1, $2)