SES: confirm cutover complete #79

Depends on the soak-test issue.

• Magic-link email flows entirely through SES
• No send errors in the logs across the soak window
• SendGrid remains configured but unused

This issue is the explicit gate before the cleanup issue removes the SendGrid path — and, because the SENDGRID_API_KEY is believed to be shared with the planner apps, an input to planner's SendGrid teardown (codebar/planner#2933): only once planner has confirmed its own cutover (codebar/planner#2930) can the shared key be revoked.