Email alerts & digests

Starter email on failure; weekly digest when SMTP is configured.

• Starter: email alerts to org admins when a scheduled drill fails.
• Pro: per-workflow recipient lists and pass/fail toggles.
• Weekly digest: fleet summary (last 7 days RTO, pass rate) when API SMTP env is set.

Evidence attachments in email are links to the vault (JSON / Markdown / PDF), not raw database dumps.