Create an async CSV import job

Accepts multipart/form-data with a file field (the CSV) and an optional block_ttl_days (integer >0, default 30). Validates:

• content ≤ 25 MiB, else 413
• row count ≤ 250 000, else 413
• header-only / all-blank / undetectable provider → 400 Returns 202 with the import record (status pending); an Oban worker (EmailBlockImportWorker, max_attempts 3) transitions pending → processing → completed | failed.

Native Telnyx exports are detected by the stable first-12-column header signature (id … group_id) and are restored with their original from, domain_id, group_id, source, status, expires_at, plus bounce_category, dsn_code, and meta when present (scope is re-derived from domain_id/from; the exported scope cell must be a valid enum value). Lifecycle changes reconcile through the same create path as the API: a row already in the requested state restores its mutable backup fields without a new audit event, and a real transition (e.g. tombstone → active) appends the matching lifecycle event. block_ttl_days is not applied to native rows — their exported expires_at is preserved verbatim.

Competitor and generic imports (SendGrid / Mailgun / SES / generic) remain account-scoped (from, domain_id, group_id, scope are not read) and block_ttl_days applies only to imported manual_block rows; other reasons get expires_at: nil. Provider is auto-detected from the CSV header (sendgrid / mailgun / ses / generic).