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. 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).