Get Desk Contact Call Target

Resolve the dialable number for one contact, after the opt-out check.

ALWAYS 200. "No number on file" and "this person opted out" are ordinary answers carried in the payload as is_dialable: false with a reason, not errors - a caller that treats a non-200 as the only failure mode will dial people who asked not to be called.

POST rather than GET is the app's choice: it consults the suppression ledger, and pressing Call is an intent to contact someone, which should not be firable by a prefetch. It resolves and writes nothing, which is why it is tiered a READ.