Implement real SMTP delivery and transactional email auth flow

Email delivery:

- Replaced logging-only email sender with aiosmtplib SMTP implementation.

- Added provider mode switch via EMAIL_PROVIDER (log/smtp).

- Added TLS/SSL and timeout controls for SMTP transport.

Auth registration flow:

- Made register/resend/reset email flows transactional with rollback on delivery failure.

- Return 503 when verification/reset email cannot be delivered.

Configuration:

- Extended settings and env templates for EMAIL_PROVIDER, SMTP_USE_SSL, SMTP_TIMEOUT_SECONDS.

- Updated docker-compose environment mapping for new SMTP variables.

.env.docker.example

@@ -40,7 +40,10 @@ SMTP_HOST=mailpit
 SMTP_PORT=1025
 SMTP_USERNAME=
 SMTP_PASSWORD=
+EMAIL_PROVIDER=log
 SMTP_USE_TLS=false
+SMTP_USE_SSL=false
+SMTP_TIMEOUT_SECONDS=10
 SMTP_FROM_EMAIL=no-reply@benyamessenger.local
 MAILPIT_SMTP_PORT=1025
 MAILPIT_UI_PORT=8025