Function prepare_search_query 

pub fn prepare_search_query(raw: &str) -> Result<PreparedQuery>

Build a safe FTS5 MATCH string and ranking tokens from raw user input.

Strategy:

1. Tokenize (punctuation-safe)
2. Prefer significant (non-stopword) tokens
3. Single token → exact quoted match
4. Multiple tokens → OR between them for natural-language recall (documents rarely contain every filler word from a full question)

Errors

Returns BrainError::FtsQuery when no usable token remains.