Fix migrate.sh auth failure by overriding built-in script

The Supabase postgres image includes a migrate.sh that tries to connect
as supabase_admin without proper credentials. Override it with an empty
script since migrations are handled by init.sql.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

supabase/docker/migrate.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+# Override the default Supabase migrate.sh
+# Our migrations are handled via init.sql which runs as postgres user
+echo "Skipping built-in migrate.sh - migrations handled by init.sql"
+exit 0