Sends an approved template to many numbers, now or at scheduled_at
(without a UTC offset it is Bangladesh time). The upper-bound cost is held from
your balance and settled when the campaign is sent; cancelling releases it.

Recipients — at most 1,000 per call (split larger lists into several
campaigns), as a JSON recipients array or a CSV file (multipart):

• static template: ["01712345678", "01812345678"]
• dynamic template: [{"to": "01712345678", "values": ["Rahim"]}] — one value per ##variable##, in order
• CSV: column 1 = number, following columns = variable values; header row skipped

Invalid / duplicate rows are skipped (skipped_rows). Send an Idempotency-Key
header to make retries safe — a repeat within 24 h returns the original campaign.