refactor: split dbx package into domain files

Reorganize monolithic dbx.go and utility.go into focused files by
concern, add package doc.go, and gitignore coverage.out.

Co-authored-by: Cursor <cursoragent@cursor.com>

query.go

@@ -0,0 +1,49 @@
+package dbx
+
+import "errors"
+
+// Fetch queries the database and returns the result as a slice. If the query
+// returns no rows, it returns an empty slice and no error.
+func Fetch[T any](sqlo Queryable, stmt Statement) ([]*T, error) {
+	var result []*T
+	if err := stmt.Query(sqlo, &result); err != nil && !errors.Is(err, ErrNoRows) {
+		return nil, err
+	}
+	return result, nil
+}
+
+// MustFetch queries the database and returns the result as a slice. If the query
+// returns no rows, it returns an empty slice and the desired error.
+func MustFetch[T any](sqlo Queryable, stmt Statement, notFoundErr error) ([]*T, error) {
+	result, err := Fetch[T](sqlo, stmt)
+	if err != nil {
+		return nil, err
+	}
+	if len(result) == 0 {
+		return nil, notFoundErr
+	}
+	return result, nil
+}
+
+// FetchOne queries the database and returns a single result. If the query
+// returns no rows, it returns nil and no error.
+func FetchOne[T any](sqlo Queryable, stmt Statement) (*T, error) {
+	result, err := Fetch[T](sqlo, stmt)
+	if err != nil {
+		return nil, err
+	}
+	if len(result) == 0 {
+		return nil, nil
+	}
+	return result[0], nil
+}
+
+// MustFetchOne queries the database and returns a single result. If the query
+// returns no rows, it returns nil and the desired error.
+func MustFetchOne[T any](sqlo Queryable, stmt Statement, notFoundErr error) (*T, error) {
+	result, err := MustFetch[T](sqlo, stmt, notFoundErr)
+	if err != nil {
+		return nil, err
+	}
+	return result[0], nil
+}