feat: full boolean search — OR, NOT, parentheses

[arrrnav]

Currently only AND is supported (implicit and explicit). Extend the query parser and retrieval logic to support the full boolean operator set.

Planned operators

• OR — union of posting lists with union-based scoring
• NOT — subtract a posting list from the candidate set
• Parentheses — grouped sub-expressions with correct operator precedence

Scope

• Backend: query parser + retrieval changes in index_creation/searcher.py
• Frontend: update the search hint on the landing page to reflect new syntax