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/db/proxy_test.go

@@ -0,0 +1,162 @@
+//go:build test
+
+package db
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// =============================================================================
+// ContextWithTx / TxFromContext round-trip
+// =============================================================================
+
+func TestContextWithTx_RoundTrip(t *testing.T) {
+	closePool()
+	resetEnv()
+	err := Connect()
+	require.NoError(t, err)
+	defer closePool()
+
+	ctx := context.Background()
+	pool := Conn.Pool()
+
+	tx, err := pool.Begin(ctx)
+	require.NoError(t, err)
+	defer tx.Rollback(ctx) //nolint:errcheck
+
+	txCtx := ContextWithTx(ctx, tx)
+	extracted := TxFromContext(txCtx)
+
+	assert.NotNil(t, extracted, "TxFromContext should return a non-nil tx")
+	assert.Equal(t, tx, extracted, "extracted tx should be the same as the one stored")
+}
+
+// =============================================================================
+// TxFromContext with no transaction in context
+// =============================================================================
+
+func TestTxFromContext_NoTx(t *testing.T) {
+	ctx := context.Background()
+	tx := TxFromContext(ctx)
+	assert.Nil(t, tx, "TxFromContext should return nil when no tx in context")
+}
+
+// =============================================================================
+// PoolProxy.Exec routes through context transaction
+// =============================================================================
+
+func TestPoolProxy_Exec_RoutesThroughTx(t *testing.T) {
+	closePool()
+	resetEnv()
+	err := Connect()
+	require.NoError(t, err)
+	defer closePool()
+
+	ctx := context.Background()
+	pool := Conn.Pool()
+
+	tx, err := pool.Begin(ctx)
+	require.NoError(t, err)
+	defer tx.Rollback(ctx) //nolint:errcheck
+
+	txCtx := ContextWithTx(ctx, tx)
+
+	_, err = Conn.Exec(txCtx, "CREATE TEMP TABLE test_exec_routing (id INT PRIMARY KEY)")
+	require.NoError(t, err)
+
+	_, err = Conn.Exec(txCtx, "INSERT INTO test_exec_routing VALUES (1)")
+	require.NoError(t, err)
+
+	var count int
+	err = Conn.QueryRow(txCtx, "SELECT COUNT(*) FROM test_exec_routing").Scan(&count)
+	require.NoError(t, err)
+	assert.Equal(t, 1, count)
+
+	err = tx.Rollback(ctx)
+	require.NoError(t, err)
+
+	_, err = Conn.Exec(ctx, "SELECT COUNT(*) FROM test_exec_routing")
+	assert.Error(t, err, "expected error querying temp table after rollback")
+}
+
+// =============================================================================
+// PoolProxy.QueryRow routes through context transaction
+// =============================================================================
+
+func TestPoolProxy_QueryRow_RoutesThroughTx(t *testing.T) {
+	closePool()
+	resetEnv()
+	err := Connect()
+	require.NoError(t, err)
+	defer closePool()
+
+	ctx := context.Background()
+	pool := Conn.Pool()
+
+	tx, err := pool.Begin(ctx)
+	require.NoError(t, err)
+	defer tx.Rollback(ctx) //nolint:errcheck
+
+	txCtx := ContextWithTx(ctx, tx)
+
+	_, err = Conn.Exec(txCtx, "CREATE TEMP TABLE test_queryrow_routing (id INT PRIMARY KEY, val TEXT)")
+	require.NoError(t, err)
+
+	_, err = Conn.Exec(txCtx, "INSERT INTO test_queryrow_routing VALUES (42, 'routed')")
+	require.NoError(t, err)
+
+	var id int
+	var val string
+	err = Conn.QueryRow(txCtx, "SELECT id, val FROM test_queryrow_routing WHERE id = 42").Scan(&id, &val)
+	require.NoError(t, err)
+	assert.Equal(t, 42, id)
+	assert.Equal(t, "routed", val)
+
+	err = tx.Rollback(ctx)
+	require.NoError(t, err)
+
+	err = Conn.QueryRow(ctx, "SELECT id, val FROM test_queryrow_routing WHERE id = 42").Scan(&id, &val)
+	assert.Error(t, err, "expected error querying temp table via pool after rollback")
+}
+
+// =============================================================================
+// PoolProxy.Begin returns a real transaction
+// =============================================================================
+
+func TestPoolProxy_Begin_ReturnsTx(t *testing.T) {
+	closePool()
+	resetEnv()
+	err := Connect()
+	require.NoError(t, err)
+	defer closePool()
+
+	ctx := context.Background()
+
+	tx, err := Conn.Begin(ctx)
+	require.NoError(t, err)
+	assert.NotNil(t, tx)
+
+	err = tx.Rollback(ctx)
+	require.NoError(t, err)
+}
+
+// =============================================================================
+// PoolProxy.Ping works
+// =============================================================================
+
+func TestPoolProxy_Ping_Works(t *testing.T) {
+	closePool()
+	resetEnv()
+	err := Connect()
+	require.NoError(t, err)
+	defer closePool()
+
+	ctx := context.Background()
+
+	err = Conn.Ping(ctx)
+	assert.NoError(t, err)
+}