fix(tests): skip postgres tests when psycopg is not installed

Added pytest.importorskip('psycopg') to tests/unit/test_postgres_backend.py to properly skip tests when the optional pgvector dependency group is not installed.

This fixes a test collection error that was preventing pytest from running.

Author: visahak

tests/unit/test_postgres_backend.py

@@ -7,6 +7,9 @@
 import pytest
 from unittest.mock import Mock, MagicMock, patch
 
+# Skip all tests in this module if psycopg is not installed
+pytest.importorskip("psycopg")
+
 from evolve.backend.postgres import PostgresEntityBackend
 from evolve.schema.core import Entity, Namespace, RecordedEntity
 from evolve.schema.conflict_resolution import EntityUpdate