Fix DATABASE_URL not exported before migrate_db in start.sh

wait_for_pg() set DATABASE_URL via := default syntax but never exported it,
so Python child processes (manage.py migrate, waffle_switch) could not read
it from the environment. The .env file does not exist yet at that point —
write_env_file runs later — so decouple raised UndefinedValueError.

Add 'export DATABASE_URL' immediately after the default is resolved in
wait_for_pg(), which is the earliest point in the startup sequence where
the value is known and already used by configure_pg_timezone right after.

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

docker/startup_scripts/start.sh

@@ -141,6 +141,7 @@ write_env_file() {
 
 wait_for_pg() {
   : "${DATABASE_URL:=postgresql://sapl:sapl@sapldb:5432/sapl}"
+  export DATABASE_URL
   log "Waiting for Postgres..."
   /bin/bash wait-for-pg.sh "$DATABASE_URL"
 }