chore(email): decommission SMTP2GO references — local MTA only

SMTP2GO is no longer used: Listmonk relays through the local Postfix MTA
(172.18.0.1:25 from the Docker network), which DKIM-signs and delivers
direct-to-recipient-MX; transactional mail goes through Carbonio. Verified
zero smtp2go in any live container env + postfix has no external relayhost.

Removed the stale references so a rebuild/new dev can't re-introduce it:
- api/src/config.ts: SMTP_HOST default mail.smtp2go.com -> co.carrierone.com
- scripts/workers/crypto_payment_worker.py: same default fix
- infra/ansible all.yml: listmonk_smtp_* now 172.18.0.1:25, no auth (+comment)
- app.env.j2 / email.ts / crm.md / go-live-todo.md / architecture.svg: docs

scripts/workers/crypto_payment_worker.py

@@ -809,7 +809,7 @@ automate this process.
             import smtplib
             from email.mime.text import MIMEText
 
-            smtp_host = os.environ.get("SMTP_HOST", "mail.smtp2go.com")
+            smtp_host = os.environ.get("SMTP_HOST", "co.carrierone.com")
             smtp_port = int(os.environ.get("SMTP_PORT", "587"))
             smtp_user = os.environ.get("SMTP_USER", "")
             smtp_pass = os.environ.get("SMTP_PASS", "")