Add Store-level unit tests for SQLite and PostgreSQL backends

Add shared test functions in store_test.go covering all Store interface
operations (users, tokens, access control, tiers, billing, stats, etc.)
with per-backend wrappers in store_sqlite_test.go and store_postgres_test.go
following the webpush test pattern.

Fix broken isUniqueConstraintError() which used incorrect interface
assertions instead of string matching for SQLite/PostgreSQL errors.

user/store_sqlite_test.go

@@ -0,0 +1,180 @@
+package user_test
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"heckel.io/ntfy/v2/user"
+)
+
+func newTestSQLiteStore(t *testing.T) user.Store {
+	store, err := user.NewSQLiteStore(filepath.Join(t.TempDir(), "user.db"), "")
+	require.Nil(t, err)
+	t.Cleanup(func() { store.Close() })
+	return store
+}
+
+func TestSQLiteStoreAddUser(t *testing.T) {
+	testStoreAddUser(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreAddUserAlreadyExists(t *testing.T) {
+	testStoreAddUserAlreadyExists(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreRemoveUser(t *testing.T) {
+	testStoreRemoveUser(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreUserByID(t *testing.T) {
+	testStoreUserByID(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreUserByToken(t *testing.T) {
+	testStoreUserByToken(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreUserByStripeCustomer(t *testing.T) {
+	testStoreUserByStripeCustomer(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreUsers(t *testing.T) {
+	testStoreUsers(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreUsersCount(t *testing.T) {
+	testStoreUsersCount(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreChangePassword(t *testing.T) {
+	testStoreChangePassword(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreChangeRole(t *testing.T) {
+	testStoreChangeRole(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTokens(t *testing.T) {
+	testStoreTokens(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTokenChangeLabel(t *testing.T) {
+	testStoreTokenChangeLabel(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTokenRemove(t *testing.T) {
+	testStoreTokenRemove(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTokenRemoveExpired(t *testing.T) {
+	testStoreTokenRemoveExpired(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTokenRemoveExcess(t *testing.T) {
+	testStoreTokenRemoveExcess(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTokenUpdateLastAccess(t *testing.T) {
+	testStoreTokenUpdateLastAccess(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreAllowAccess(t *testing.T) {
+	testStoreAllowAccess(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreAllowAccessReadOnly(t *testing.T) {
+	testStoreAllowAccessReadOnly(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreResetAccess(t *testing.T) {
+	testStoreResetAccess(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreResetAccessAll(t *testing.T) {
+	testStoreResetAccessAll(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreAuthorizeTopicAccess(t *testing.T) {
+	testStoreAuthorizeTopicAccess(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreAuthorizeTopicAccessNotFound(t *testing.T) {
+	testStoreAuthorizeTopicAccessNotFound(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreAuthorizeTopicAccessDenyAll(t *testing.T) {
+	testStoreAuthorizeTopicAccessDenyAll(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreReservations(t *testing.T) {
+	testStoreReservations(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreReservationsCount(t *testing.T) {
+	testStoreReservationsCount(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreHasReservation(t *testing.T) {
+	testStoreHasReservation(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreReservationOwner(t *testing.T) {
+	testStoreReservationOwner(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTiers(t *testing.T) {
+	testStoreTiers(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTierUpdate(t *testing.T) {
+	testStoreTierUpdate(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTierRemove(t *testing.T) {
+	testStoreTierRemove(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreTierByStripePrice(t *testing.T) {
+	testStoreTierByStripePrice(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreChangeTier(t *testing.T) {
+	testStoreChangeTier(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStorePhoneNumbers(t *testing.T) {
+	testStorePhoneNumbers(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreChangeSettings(t *testing.T) {
+	testStoreChangeSettings(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreChangeBilling(t *testing.T) {
+	testStoreChangeBilling(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreUpdateStats(t *testing.T) {
+	testStoreUpdateStats(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreResetStats(t *testing.T) {
+	testStoreResetStats(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreMarkUserRemoved(t *testing.T) {
+	testStoreMarkUserRemoved(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreRemoveDeletedUsers(t *testing.T) {
+	testStoreRemoveDeletedUsers(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreAllGrants(t *testing.T) {
+	testStoreAllGrants(t, newTestSQLiteStore(t))
+}
+
+func TestSQLiteStoreOtherAccessCount(t *testing.T) {
+	testStoreOtherAccessCount(t, newTestSQLiteStore(t))
+}