Update campaign

Update fields on a campaign (ID & account_id are provided by path/context; not in body).

Request body fields (wrapped in campaign object):

• name (string, required) - Human-readable campaign name.
• account_id (string, required) - Target account ID.
• status (string, optional) - DRAFT | RUN | PAUSE | DONE. Defaults to DRAFT.
• seats (array of strings, required) - Seat IDs to run this campaign.
• schedule (object, required) - Day-of-week schedule. Each day key maps to [enabled (bool), start_hour (int), end_hour (int)].
• timezone (integer, optional) - UTC offset in minutes (e.g. -300 = UTC-5).
• campaign_id (string, optional) - Supply to create with a specific ID; omit to auto-generate.