Add FTS5 hybrid search (keyword + semantic)

[jonyoder]

The sqlite-vec database already uses SQLite. Adding an FTS5 table alongside the vector table would enable hybrid search — keyword matching for exact function/type names combined with semantic vector search for conceptual queries.

The code_items metadata table is already there; just need to create an FTS5 virtual table indexing the same data.

This would improve results for queries like exact function names (where keyword match is better) while keeping semantic search for conceptual queries.