=== Section 8: Invoice generator - crypto strong + collision + uniqueness ===
49:        self.rng = rng or secrets.SystemRandom()
59:        Cryptographically strong randomness via SystemRandom.
107:                return secrets.token_hex(n)

=== Section 17: DLQ / retry / pause/resume/cancel ===
1:"""Send task: real Gmail API/SMTP send with retry, backoff, DLQ, pause/cancel, rate limits.
5:  temporary errors -> retry with exponential backoff + jitter, dead-letter after max attempts
403:                          detail=f"dead-letter after {cr.attempt_count} attempts: {error[:300]}"))
406:        _push_dlq(campaign.id, cr.id, error)
408:        return {"ok": False, "status": "dead_letter", "reason": error[:200]}

=== Section 19: SSRF protection ===