Function is_valid_database 

pub fn is_valid_database(db_path: &Path) -> bool

Check if a database directory is valid and complete

A valid database must contain:

• metadata.json (model info, dimensions)
• data.mdb file (LMDB vector embeddings) - directly in db folder
• fts/ directory (full-text search index)

Returns true if the database appears valid, false otherwise.