mailserver: decommission SendGrid

Remove leftover SendGrid references after the Brevo migration was completed:

- Delete TF `cloudflare_record.mail_domainkey` (TXT at `s1._domainkey`,
  SendGrid-era DKIM, hidden behind the SendGrid CNAME but would re-emerge
  once the CNAME is removed).
- Clean up commented-out `smtp.sendgrid.net` relayhost references and the
  `# For sendgrid` comment on `sasl_passwd` in the mailserver module.

DNS records deleted out-of-band (not TF-managed):
- CF: `s1._domainkey CNAME` + `s2._domainkey CNAME` → sendgrid.net (manual entries)
- Technitium internal `viktorbarzin.me`: `em7107`, `s1._domainkey`,
  `s2._domainkey` CNAMEs → sendgrid.net

Verified end-to-end mail flow unaffected (Brevo outbound + IMAP receive,
roundtrip 20.4s — identical to baseline). Active DKIM (`mail._domainkey`
local + `brevo1/brevo2._domainkey` Brevo) untouched.

stacks/cloudflared/modules/cloudflared/cloudflare.tf

@@ -145,16 +145,6 @@ resource "cloudflare_record" "mail_mx" {
 }
 
 
-resource "cloudflare_record" "mail_domainkey" {
-  content  = "\"v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDIDLB8mhAHNqs1s6GeZMQHOxWweoNKIrqo5tqRM3yFilgfPUX34aTIXNZg9xAmlK+2S/xXO1ymt127ZGMjnoFKOEP8/uZ54iHTCnioHaPZWMfJ7o6TYIXjr+9ShKfoJxZLv7lHJ2wKQK3yOw4lg4cvja5nxQ6fNoGRwo+mQ/mgJQIDAQAB\""
-  name     = "s1._domainkey.viktorbarzin.me"
-  proxied  = false
-  ttl      = 1
-  type     = "TXT"
-  priority = 1
-  zone_id  = var.cloudflare_zone_id
-}
-
 resource "cloudflare_record" "mail_spf" {
   # Brevo replaced Mailgun as the outbound relay on 2026-04-12 (see docs/architecture/mailserver.md).
   # Soft-fail (~all) is intentional during cutover — revisit once relay delivery is stable.