feat(starttls): remove opportunistic value

This commit removes the `Opportunistic` value from the struct `StartTls`.
This value was strictly equivalent to `Always` and could potentially
cause confusion.

README.md

@@ -321,7 +321,7 @@ backend/
 
 The contact form supports multiple SMTP configurations:
 - **Implicit TLS (SMTPS)** - typically port 465
-- **STARTTLS (Always/Opportunistic)** - typically port 587
+- **STARTTLS (Always)** - typically port 587
 - **Unencrypted** (for local dev) - with or without authentication
 
 The `SmtpTransport` is built dynamically from `EmailSettings` based on