Create a CRM activity after a call

1. Subscribe to the call-completed event.
2. Verify the signature and deduplicate on the event id.
3. Match the caller to a CRM contact by E.164 number; create one if absent.
4. Re-fetch the call from the API rather than trusting the payload as canonical.
5. Write the activity, storing the Talki call id so a redelivery updates rather than duplicates.
6. On startup, sweep with updated_after to catch anything missed while down.