Enhance vector search migration to conditionally handle pgvector extension

Commit notes

- Added checks to ensure the pgvector extension is available before creating the embedding_vector column and associated indexes. - Wrapped operations in DO blocks to gracefully skip setup if pgvector is not installed, allowing the application to fall back on JavaScript for similarity search. - Included notices for successful operations and backfilling of existing embeddings. - Updated comments for clarity on the migration's behavior with and without pgvector.

This improves the robustness of the migration process and ensures compatibility with environments lacking the pgvector extension.