Add pgvector integration tests for embeddings similarity search

[anchapin]

Summary

Missing integration tests for vector similarity search functionality with PostgreSQL pgvector extension.

Location

backend/src/tests/unit/test_crud_embeddings.py:193

Context

# TODO: Add a test for find_similar_embeddings that runs only if connected to a real Postgres with pgvector.
# This would typically involve a separate conftest.py for integration tests or a way to mark tests
# based on DB capabilities.
# For example: @pytest.mark.integration_postgres

Requirements

• Create integration test for find_similar_embeddings function
• Add pytest markers for PostgreSQL/pgvector tests
• Set up test configuration to detect pgvector capability
• Add proper test fixtures for vector similarity testing
• Ensure tests skip gracefully when pgvector is unavailable

Priority

Medium - Important for testing vector search functionality