Get the call's active supervision

Rings a supervisor onto an answered, in-progress call in `listen` (hear only), `whisper` (hear both sides, and be heard by the agent only) or `barge` (hear and speak to both sides) mode. Only an ordinary two-party call can be supervised — AI-agent calls, conference sessions and callers still waiting in a queue all return `calls.unsupported_call_type`. A `client` or `sip` target that is not registered, or a `client` target that resolves to a different workspace, returns `calls.supervisor_unreachable`. A muted supervisor is never heard by the agent or caller, even in `whisper` or `barge` — muting the leg (locally, on the supervisor's own device) always wins over the mode. One supervisor per call, enforced by a database constraint: starting a second one returns `calls.supervision_in_progress` naming the current supervisor. If a `DELETE` on this same call raced the request and won, this returns `calls.supervision_stopped`. Mode changes only ever happen through this API — the PBX switches the leg via an internal command, so a supervisor's own handset DTMF does no such thing, and every `mode_changed` event this produces carries `source: "api"`. Billed per minute from the supervisor leg's connect to its end: KES 0.50/min for a `client` or `sip` supervisor, the workspace's normal outbound rate for a `phone` supervisor (rated off the supervised call's own number). A ringing or failed leg is never billed. Progress and billing both arrive asynchronously as `call.supervision.*` webhooks; this call only returns the leg's initial `connecting` state. **Who can call this:** a signed-in workspace owner or admin, or a member holding the Supervisor licence — or an API key or OAuth token with the `calls.supervise` scope. A device (handset) credential can never supervise calls.