Add ALTER TABLE migration for auto_signup column on existing ClickHouse

The CREATE TABLE IF NOT EXISTS won't add new columns to an existing table.
Add 05-oidc-auto-signup.sql with ALTER TABLE ADD COLUMN IF NOT EXISTS and
register it in ClickHouseConfig startup schema + test init.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clickhouse/init/05-oidc-auto-signup.sql

@@ -0,0 +1 @@
+ALTER TABLE oidc_config ADD COLUMN IF NOT EXISTS auto_signup Bool DEFAULT true;