Add HNSW vector index for scalable similarity search

Commit notes

- Add migration 0018_vector_search_index.sql with HNSW index - Ensures pgvector extension is enabled - Creates embedding_vector column with sync trigger from embedding array - Adds HNSW index with cosine distance for O(log n) search complexity - Adds partial indexes for project-scoped and user-scoped searches - Includes detailed performance documentation

This makes similarity search scale to thousands/millions of samples without performance degradation.

Co-authored-by: armin.naimi <[email redacted]>