fix: add pg_conn fixture for integration tests (#84)

Adds the missing `pg_conn` session-scoped pytest fixture to conftest.py.
Without it, the 6 integration tests in test_evolution_migration.py fail
immediately with `fixture 'pg_conn' not found` whenever POSTGRES_HOST is set.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -8,7 +8,41 @@ responsible for binding models to its own in-memory database.
 
 import os
 
+import psycopg2
+import pytest
+
 os.environ["DATABASE_TYPE"] = "postgresql"
 # Provide dummy credentials so PooledPostgresqlDatabase can be instantiated
 # without raising a configuration error (it will not actually be used).
 os.environ.setdefault("POSTGRES_PASSWORD", "test-dummy")
+
+
+@pytest.fixture(scope="session")
+def pg_conn():
+    """Live PostgreSQL connection for integration tests.
+
+    Requires POSTGRES_HOST to be set; runs the migration before yielding the
+    connection so that all integration tests start from a migrated schema.
+    The connection is rolled back and closed at session teardown so that the
+    migration is not persisted to the dev database between test runs.
+    """
+    conn = psycopg2.connect(
+        host=os.environ["POSTGRES_HOST"],
+        dbname=os.environ.get("POSTGRES_DB", "pd_master"),
+        user=os.environ.get("POSTGRES_USER", "pd_admin"),
+        password=os.environ["POSTGRES_PASSWORD"],
+        port=int(os.environ.get("POSTGRES_PORT", "5432")),
+    )
+    conn.autocommit = False
+    migration_path = os.path.join(
+        os.path.dirname(__file__),
+        "..",
+        "migrations",
+        "2026-03-12_add_evolution_tables.sql",
+    )
+    with open(migration_path) as fh:
+        conn.cursor().execute(fh.read())
+    conn.commit()
+    yield conn
+    conn.rollback()
+    conn.close()