Composable filter pipeline that processes SavingItem slices through
chained predicates: BOGO detection (category match), exact category
match, substring department match, and keyword search across title
and description fields. All text matching is case-insensitive.
Includes utility functions for HTML entity unescaping (CleanText),
nil-safe string pointer dereferencing (Deref), and case-insensitive
slice membership (ContainsIgnoreCase). An optional limit truncates
results after all filters are applied. Tests cover each filter in
isolation, combined filters, nil field safety, and the Categories
aggregation helper.
