Start a session data backfill (super_admin)

Start an in-memory background backfill for one WhatsApp session, re-pulling the directly fetchable WhatsApp data for that number. The job pulls only the data WhatsApp exposes through a direct fetch: **cached contacts** and **joined group metadata/members**. Ordinary chat history is **not** part of a backfill — message history arrives asynchronously through WhatsApp `HistorySync` events, and there is no generic "fetch all messages" API. ### Asynchronous behavior - Returns **202 Accepted** immediately with the created/running `BackfillJob`; the work continues in the background. - Poll `GET /admin/sessions/{session}/backfill` to observe progress and completion. ### Idempotency / concurrency - At most **one** backfill may run per session at a time. If a backfill is already in progress for this session, the request is rejected with **409** rather than starting a second one — it is **not** a no-op merge into the existing job. ### Preconditions - Requires the platform **super_admin** role (login JWT only; api-keys and org roles are rejected with 403). - The session must exist. ### Errors - `forbidden` (403) — caller is not a platform super_admin. - `not_found` (404) — no session with the given id exists. - `conflict` (409) — a backfill is already running for this session; wait for it to finish (poll the status endpoint) before retrying. - `not_implemented` (501) — backfill is unavailable in this build/configuration for the requested session.

curl -X POST "https://example.com/api/v1/admin/sessions/01HZX9Q2K7:backfill"

{  "contacts": 128,  "error": "session not connected",  "finishedAt": 1719662460000,  "groupMembers": 342,  "groups": 14,  "id": "01J9...",  "organizationId": "org_01J9ABC...",  "sessionId": "01J9ZX8K2QHV0M3T6R7P4N5W8C",  "startedAt": 1719662400000,  "status": "succeeded"}

{  "error": {    "code": "not_found",    "details": {      "property1": null,      "property2": null    },    "message": "session not found"  }}