Follow-up from PR #297 clawpatch review.

External webhook duplicate suppression checks for an existing claw before insert, but concurrent deliveries can race and create duplicate claws for the same factory/external_trigger_id.

Scope:

• Use the factory_triggers claim/complete mechanism or a DB uniqueness/claim row for external events.
• Treat duplicate claims as idempotent success.
• Add a concurrent delivery regression test.