fix: preserve subscription identity across re-syncs
register_subscription was generating a random SID as the name fallback
when the garden didn't provide one. This defeated the upsert on
(garden_id, org_id, name), causing every re-sync to create a new
subscription record and orphan the subscriptions_deployments links.
Now falls through to create_subscription's existing default of
seed_name, which is stable across re-syncs.
sow-134
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>