refactor logic in get_semantic_similarity_match()

[K-Schubert]

Description

Simplify sql query (l.132-144):

    if max_results == -1:
        matches = await conn.fetch(f"""
            SELECT question, answer, url,  1 - (embedding {similarity_metric_symbol} '{question_embedding}') AS {similarity_metric}
            FROM faq_embeddings
            ORDER BY {similarity_metric} desc
        """)
    else:
        matches = await conn.fetch(f"""
            SELECT question, answer, url,  1 - (embedding {similarity_metric_symbol} '{question_embedding}') AS {similarity_metric}
            FROM faq_embeddings
            ORDER BY {similarity_metric} desc
            LIMIT $1
        """, max_results)

Instead of setting "no limit" to "-1", simply refactor logic to accept any limit and specify acceptable values in documentation.