Third-party integration

# Connect [service] to [do what] ## IntentAs a [role], I want [service] connected so that [outcome]. ## Context- Service docs: [link to the API docs]- Account and keys: [where the sandbox credentials are; never in the ticket]- Our side: [`path/to/integrations/`, an existing integration to copy] ## Behaviour1. An admin connects [service] under [Settings → Integrations] with [what they enter].2. When [event], we [send or read what].3. When [service] is down or refuses, [what the person sees and what is retried]. ## Edge cases- Expired credentials, rate limits, duplicate events, partial failures ## Constraints- Secrets are stored server-side only and never shown again.- Nothing is sent to [service] until an admin connects it. ## Out of scope- [Other directions of sync, other services] ## Open questions- [Which test account and credentials can be used, and what are the rate limits? Remove it when settled.] ## Acceptance checks- An admin can connect and test [service] with sandbox credentials- When [event], [service] receives [what] exactly once- When [service] fails, the person sees "[message]" and nothing is lost- Credentials are never returned by any endpoint