Dispatch a push notification of an existing category to a user of the authenticated organisation. Requires the manual-send feature to be enabled. The server-level category configuration and the recipient own preference still apply: when either suppresses the notification the response reports targetCount 0 with the reason. Devices registered against several signers of the same user are dispatched to once, so targetCount counts distinct destinations. Rate limited per caller to the configured PUSH_MANUAL_SEND_RATE_LIMIT_PER_MINUTE in a fixed 60s window; the window is per API instance, so with N instances the effective ceiling is N times the configured limit. Exceeding it returns 429 with meta.retryAfterSeconds. The notification is handed to the delivery pipeline before the response: when that dispatch is unavailable or fails, the call returns 503 rather than a positive targetCount.