feat(membership,client): HTTP admin-only users API

Close the last control-plane asymmetry: rooms had a signed HTTP surface
but users were only manageable via the local CLI or direct store access.
Add admin-only HTTP endpoints, symmetric with rooms, executed against the
same privileged store the server already serves (SQLite single-node, the
replicated JetStream KV in cluster) — no new KV connection, no internal
identity, so the admin panel can manage the allowlist by signing as an
admin instead of needing --db / direct KV access.

Endpoints (all behind requireAdmin, on top of the existing
signature+nonce+TLS+enforce middleware):
  - GET  /users                    list the full allowlist (incl. revoked)
  - POST /users                    add {sign_pub, handle, role}
  - POST /users/{signpub}/revoke   revoke (status flip, no hard delete)

requireAdmin is default-deny with no dev relaxation: it allows a request
only when the authenticated signer is confirmed by the store as an active
admin; any other case (no signer, non-admin, revoked, store error) is 403,
fail-closed. The request context now also carries the signer's sign_pub
hex, because the endpoint id is a one-way hash of the key and cannot be
reversed to look the signer up in the allowlist.

Validation/idempotency mirror the CLL: sign_pub must be 64-hex, role must
be admin|member (empty defaults to member), re-adding an existing key is a
409 that leaves the row untouched. The hex check is unified into
membership.ValidateSignPubHex, reused by the CLI and the handlers.

pkg/client gains ListUsers/AddUser/RevokeUser (flat UserInfo type) signed
via doJSON, so the panel plugs in directly.

Tests: non-admin -> 403 on all three endpoints; admin add->list->revoke
roundtrip; validation (400 hex, 400 role, 409 re-add, row untouched); plus
a client test against an embedded membershipd under enforce.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/client/client.go

@@ -456,6 +456,23 @@ type memberRoomJSON struct {
 	Role    string     `json:"role"`
 }
 
+// userJSON mirrors the server's wire type on the admin user-management endpoints.
+type userJSON struct {
+	SignPub   string `json:"sign_pub"`
+	Handle    string `json:"handle"`
+	Role      string `json:"role"`
+	Status    string `json:"status"`
+	CreatedAt string `json:"created_at"`
+	RevokedAt string `json:"revoked_at,omitempty"`
+}
+
+// addUserReq is the POST /users body (mirror of the server type).
+type addUserReq struct {
+	SignPub string `json:"sign_pub"`
+	Handle  string `json:"handle"`
+	Role    string `json:"role"`
+}
+
 // ---- room operations ------------------------------------------------------
 
 // RoomRef is a room this peer belongs to, returned by ListMyRooms. It is the
@@ -490,6 +507,59 @@ func (c *Client) ListMyRooms() ([]RoomRef, error) {
 	return out, nil
 }
 
+// ---- user administration (admin-only) ------------------------------------
+
+// UserInfo is a bus user as returned by the admin user-management endpoints. It
+// is a flat view (no nested types) for the admin panel: the signing key
+// (lowercase hex), handle, role ("admin"|"member"), status ("active"|"revoked"),
+// and timestamps. RevokedAt is empty for an active user.
+type UserInfo struct {
+	SignPub   string
+	Handle    string
+	Role      string
+	Status    string
+	CreatedAt string
+	RevokedAt string
+}
+
+// ListUsers returns the full bus allowlist, including revoked users. The caller
+// must be signing as an admin: a non-admin signer is rejected by the server with
+// 403, surfaced here as an error.
+func (c *Client) ListUsers() ([]UserInfo, error) {
+	var resp []userJSON
+	if err := c.doJSON("GET", "/users", nil, &resp); err != nil {
+		return nil, err
+	}
+	out := make([]UserInfo, 0, len(resp))
+	for _, u := range resp {
+		out = append(out, UserInfo{
+			SignPub:   u.SignPub,
+			Handle:    u.Handle,
+			Role:      u.Role,
+			Status:    u.Status,
+			CreatedAt: u.CreatedAt,
+			RevokedAt: u.RevokedAt,
+		})
+	}
+	return out, nil
+}
+
+// AddUser registers a bus user from their Ed25519 signing public key (64-hex).
+// role is "admin" or "member" (empty defaults to member, matching the server).
+// The caller must be signing as an admin. Re-adding an already-registered key
+// returns an error (the server replies 409 and leaves the existing row
+// untouched — no silent role/status change).
+func (c *Client) AddUser(signPub, handle, role string) error {
+	return c.doJSON("POST", "/users", addUserReq{SignPub: signPub, Handle: handle, Role: role}, nil)
+}
+
+// RevokeUser revokes a bus user by their signing public key (64-hex). Revocation
+// is a status flip (no hard delete): the identity stays auditable and is denied
+// on both planes immediately. The caller must be signing as an admin.
+func (c *Client) RevokeUser(signPub string) error {
+	return c.doJSON("POST", "/users/"+signPub+"/revoke", nil, nil)
+}
+
 // newRoomKey returns 32 random bytes for a symmetric room key.
 func newRoomKey() ([]byte, error) {
 	k := make([]byte, 32)