Function fts_phrase 

fn fts_phrase(search: &str) -> Option<String>

Convert a user search string into an FTS5 phrase query for the trigram tokenizer. Returns None for empty/whitespace input. The result is wrapped in double quotes (phrase match) with internal quotes doubled per FTS5 syntax. Trigram tokenizer indexes substrings, so "foo" matches any row containing “foo” as a substring in any indexed column. Returns an FTS5 phrase for trigram MATCH, or None if the search is empty or too short (trigram needs ≥3 chars). Callers must fall back to LIKE for 1–2 char searches.