Create reach-frequency prediction

Creates a Meta ad that opens a chat with you on WhatsApp, Messenger or Instagram Direct when someone taps it. Creates a click-to-message ad; `destination` selects where the tapped ad opens a conversation: WhatsApp, the Page's Messenger inbox or the linked Instagram account's Direct inbox. `destinations` puts two or three of them on one ad set and lets Meta pick the app per viewer. The ad set is created with the matching destination_type and CONVERSATIONS optimization; the campaign objective defaults to OUTCOME_ENGAGEMENT. Supports single-creative and multi-creative shapes. Supersedes POST /v1/ads/ctwa (deprecated, equivalent to `destination: whatsapp`). Existing posts and reels are supported through `platformPostId` (alias `existingPostId`) or `objectStoryId`, either per creative or at the top level. Omit fresh media and copy for that creative. Optional `whatsappPhoneNumber` selects a number already paired with the Page (WhatsApp destination only). `accountId` is a Facebook, Instagram or Meta ads (business login) connection; `pageId` picks the Page when that connection was granted several. **Required fields per shape** (a missing one is a 400 naming it in `param`): - Always `accountId`, `adAccountId`, `name`, and exactly one of `destination` / `destinations`. - `budgetAmount` and `budgetType` unless `adSetId` or `existingCampaignId` is set, plus `endDate` for a lifetime budget. - Single image or video: `headline`, `body`, and exactly one of `imageUrl` / `video`. With `placementAssets`, `headline` and `body` only (the media comes from `placementAssets`). - `carouselCards`: `body` and the 2-10 cards; `headline` is optional, `imageUrl` / `video` / top-level `description` are a 400. - Existing post (`platformPostId` or `objectStoryId`): no copy or media fields. - `creatives[]`: the same rules per entry, with no top-level creative fields. - `welcomeMessage`, when sent: `text` plus exactly one of `prefillText` / `quickReplies`. **WhatsApp number prerequisite.** For the WhatsApp destination, link the WhatsApp number to the Facebook Page used for the ad before creating it: Meta checks the pairing at creation. Link it from the WhatsApp Business app (Settings > Business tools > Facebook & Instagram) or from the Page settings (Linked accounts > WhatsApp). Linking does not affect the number's WhatsApp connection in Zernio (Cloud API or coexistence), so no reconnect or re-sync is needed. If Meta creates a new ad set without the requested `whatsappPhoneNumber`, the 201 response carries a `warnings` entry. This can happen even when the number is linked (Meta can drop a Page's second linked number). **Idempotency:** this endpoint is not idempotent at the platform level (a blind retry creates a second campaign/ad set/ad). Send an `Idempotency-Key` header to make retries safe: the first request with a given key creates the ad and we store the response; a retry with the same key replays that exact response (with `Idempotent-Replayed: true`) instead of creating duplicates. Reusing a key with a different body returns 422; a key whose first request is still in flight returns 409 (retry after a short backoff). If the first request is cut off mid-create (for example by a function timeout), its key can stay in progress and keep returning 409 until it expires; use a new key to retry sooner, after checking whether the ad was created. Keys are scoped to your credential and expire after 24h.