refactor: reorganize project structure following go standards

internal/db/helpers.go

@@ -0,0 +1,43 @@
+package database
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"fmt"
+)
+
+func NullStringOr(n sql.NullString, fallback string) string {
+	if n.Valid && n.String != "" {
+		return n.String
+	}
+	return fallback
+}
+
+func DisplayTitle(titleEnglish, titleJapanese sql.NullString, titleOriginal string) string {
+	return NullStringOr(titleEnglish, NullStringOr(titleJapanese, titleOriginal))
+}
+
+func (r GetUserWatchListRow) DisplayTitle() string {
+	return DisplayTitle(r.TitleEnglish, r.TitleJapanese, r.TitleOriginal)
+}
+
+func BoolPtr(b sql.NullBool) *bool {
+	if !b.Valid {
+		return nil
+	}
+	return &b.Bool
+}
+
+func BeginTx(ctx context.Context, db *sql.DB) (*Queries, *sql.Tx, error) {
+	if db == nil {
+		return nil, nil, errors.New("database unavailable")
+	}
+
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to begin transaction: %w", err)
+	}
+
+	return New(tx), tx, nil
+}