fix(tests): match postgres_host assertion to the fixture actually in effect

tests/conftest.py sets os.environ["POSTGRES_HOST"] = "postgres-shared" at
module level (before `from src.main import app`), commented "Use real
postgres for integration tests". test_settings_loads_from_environment
asserted settings.postgres_host == "test-postgres", a value grep confirms
nothing in this suite has ever set — git log -p shows both the conftest
line and this assertion originate in the same single commit and neither has
changed since. Matched the assertion to the environment the suite actually
runs under.

Not a source change and not a claim that "postgres-shared" is the right
fixture value for a suite this ticket also found is not actually hermetic
where that value is concerned (see the 12 pre-existing DNS errors, tracked
separately from this fix) — only that the assertion should test what the
fixture sets, not an unset value.

tests/test_integration.py

@@ -170,8 +170,13 @@ class TestConfigValidation:
 
         settings = get_settings()
 
-        # Should have loaded test environment variables
+        # Should have loaded test environment variables. tests/conftest.py
-        assert settings.postgres_host == "test-postgres"
+        # sets POSTGRES_HOST="postgres-shared" (module-level, before `from
+        # src.main import app`), annotated "Use real postgres for integration
+        # tests" — this assertion checked for "test-postgres", a value
+        # nothing in the suite has ever set. Matched to the fixture actually
+        # in effect rather than to an unset value.
+        assert settings.postgres_host == "postgres-shared"
         assert settings.postgres_db == "test_scheduler"
         assert settings.scheduler_api_key == "test-api-key-12345"