SparkPost integration: make delivery events traceable

Email acceptance, delivery, bounce, complaint, and open tracking are separate signals.

• Authenticate sending domains and align SPF, DKIM, and DMARC.
• Use stable transmission and message identifiers.
• Verify webhooks and process duplicates safely.
• Suppress hard bounces, complaints, and opt-outs promptly.
• Keep event and receipt times separate.

Test template failure, rate limits, delayed delivery, provider retries, and tracking blocked by clients. Minimize message content and recipient data in logs.