Start a campaign

Start a campaign, transitioning it from DRAFT to RUNNING.

Prerequisites checked by the service:
- The campaign must have contacts assigned
- The campaign must have at least one verified caller ID configured
- AI-based strategies require an agent to be assigned
- Human strategies require operators to be assigned and online
- The organization wallet must have sufficient balance

Returns the campaign ID and new status on success.
Validation failures (missing contacts, missing caller IDs, invalid state
transition, insufficient balance, etc.) are returned as 400 errors with
the service message.

Authentication:
- API key with READ_WRITE permission AND the outbound-call entitlement
  (starting a campaign places outbound calls)