test: add coverage tests across backend + fix mock for PENDING checkout support

New test files cover previously untested paths across DAV, validators,
S3, Square, mw, bookings, user, and payments packages.

Includes mock fix: HoldCheckouts flag on MockClient allows tests to
pause auto-complete goroutine for testing PENDING checkout states.

Coverage: 50.4% → 65.0% (+14.6pp)

backend/handlers/user/testmain_test.go

@@ -3,11 +3,13 @@
 package user
 
 import (
+	"log"
 	"os"
 	"testing"
 
 	"crussell/db"
 	"crussell/internal/dav"
+	"crussell/internal/s3"
 	"crussell/testutils/jwt"
 	"crussell/testutils/testdb"
 )
@@ -17,6 +19,9 @@ func TestMain(m *testing.M) {
 	db.Conn = db.NewPoolProxy(pool)
 	jwt.Init()
 	dav.Service = &dav.BaseService{}
+	if err := s3.Connect(); err != nil {
+		log.Printf("WARNING: S3 not available (RUSTFS not running?), tests requiring S3 will fail: %v", err)
+	}
 	testdb.SeedBaseline(pool)
 	code := m.Run()
 	testdb.DestroyTestDatabase(pool, "crussell_test_handlers_user")