feat(server/m7-user-mgmt): admin endpoints (invites, users, promote/demote)

Five admin endpoints under existing RequireAdmin middleware:

- GET /api/admin/invites — list active + recently-redeemed
- POST /api/admin/invites — generate 24h invite, returns
  {token, expires_at, ...}. Audits ActionInviteCreate.
- DELETE /api/admin/invites/{token} — revoke unredeemed invite.
  Audits ActionInviteRevoke.
- GET /api/admin/users — list all users (id, username,
  display_name, is_admin, created_at).
- PUT /api/admin/users/{id}/admin — toggle is_admin with
  last-admin guard. Audits ActionPromoteAdmin / ActionDemoteAdmin.

Last-admin guard counts admins, refuses demotion of the sole
admin with 409 'last_admin'. Race window between count and update
is acceptable for v1 — worst case is 'no admins left,' which the
env-driven bootstrap or CLI reset can recover from. Common path
('admin demotes themselves') is now blocked.

Adds ListUsers, CountAdmins, UpdateUserAdmin sqlc queries to
users.sql.

Tests cover: invite create/list/delete round-trip, non-admin gets
403, user list, promote happy path, last-admin demotion refused,
two-admins demotion allowed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/admin_users_test.go

@@ -0,0 +1,125 @@
+package api
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/auth"
+)
+
+func newAdminUsersRouter(h *handlers) chi.Router {
+	r := chi.NewRouter()
+	r.Route("/api/admin", func(admin chi.Router) {
+		admin.Use(auth.RequireAdmin())
+		admin.Get("/users", h.handleAdminListUsers)
+		admin.Put("/users/{id}/admin", h.handleUpdateUserAdmin)
+	})
+	return r
+}
+
+func TestAdminUsers_List(t *testing.T) {
+	if os.Getenv("MINSTREL_TEST_DATABASE_URL") == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	h, pool := testHandlers(t)
+	admin := seedUser(t, pool, "listadmin", "pw", true)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/admin/users", nil)
+	req = withUser(req, admin)
+	rec := httptest.NewRecorder()
+	newAdminUsersRouter(h).ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200; body=%s", rec.Code, rec.Body.String())
+	}
+	var resp struct {
+		Users []struct{ Username string } `json:"users"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(resp.Users) == 0 {
+		t.Errorf("users list is empty; expected at least the seeded admin")
+	}
+}
+
+func TestAdminUsers_Promote(t *testing.T) {
+	if os.Getenv("MINSTREL_TEST_DATABASE_URL") == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	h, pool := testHandlers(t)
+	admin := seedUser(t, pool, "promadmin", "pw", true)
+	target := seedUser(t, pool, "promtarget", "pw", false)
+
+	body := `{"is_admin": true}`
+	req := httptest.NewRequest(http.MethodPut, "/api/admin/users/"+uuidToString(target.ID)+"/admin",
+		bytes.NewReader([]byte(body)))
+	req = withUser(req, admin)
+	rec := httptest.NewRecorder()
+	newAdminUsersRouter(h).ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200; body=%s", rec.Code, rec.Body.String())
+	}
+	var isAdmin bool
+	if err := pool.QueryRow(context.Background(),
+		"SELECT is_admin FROM users WHERE id = $1", target.ID).Scan(&isAdmin); err != nil {
+		t.Fatalf("verify: %v", err)
+	}
+	if !isAdmin {
+		t.Errorf("target not promoted")
+	}
+}
+
+func TestAdminUsers_Demote_LastAdmin_Refused(t *testing.T) {
+	if os.Getenv("MINSTREL_TEST_DATABASE_URL") == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	h, pool := testHandlers(t)
+	// Reset to ensure exactly one admin exists.
+	if _, err := pool.Exec(context.Background(), "DELETE FROM users"); err != nil {
+		t.Fatalf("reset: %v", err)
+	}
+	soleAdmin := seedUser(t, pool, "soleadmin", "pw", true)
+
+	body := `{"is_admin": false}`
+	req := httptest.NewRequest(http.MethodPut, "/api/admin/users/"+uuidToString(soleAdmin.ID)+"/admin",
+		bytes.NewReader([]byte(body)))
+	req = withUser(req, soleAdmin)
+	rec := httptest.NewRecorder()
+	newAdminUsersRouter(h).ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusConflict {
+		t.Errorf("status = %d, want 409 (last_admin guard)", rec.Code)
+	}
+}
+
+func TestAdminUsers_Demote_OtherAdmin_Allowed(t *testing.T) {
+	if os.Getenv("MINSTREL_TEST_DATABASE_URL") == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	h, pool := testHandlers(t)
+	if _, err := pool.Exec(context.Background(), "DELETE FROM users"); err != nil {
+		t.Fatalf("reset: %v", err)
+	}
+	caller := seedUser(t, pool, "caller", "pw", true)
+	target := seedUser(t, pool, "targetadmin", "pw", true)
+
+	body := `{"is_admin": false}`
+	req := httptest.NewRequest(http.MethodPut, "/api/admin/users/"+uuidToString(target.ID)+"/admin",
+		bytes.NewReader([]byte(body)))
+	req = withUser(req, caller)
+	rec := httptest.NewRecorder()
+	newAdminUsersRouter(h).ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d, want 200 (two admins exist; demoting one is fine); body=%s", rec.Code, rec.Body.String())
+	}
+}