feat: add STARTTLS support for SMTP proxy server (#876)

* feat: add STARTTLS support for SMTP proxy server

Add smtp_tls_cert and smtp_tls_key environment variables to enable
STARTTLS on the SMTP proxy server, matching existing IMAP TLS support.

Closes #249

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: add E2E tests for SMTP/IMAP STARTTLS

- Add smtp-proxy-tls service with self-signed certs in docker-compose
- Add smtp-tls.spec.ts: SMTP STARTTLS send plain/HTML/auth tests
- Add imap-tls.spec.ts: IMAP STARTTLS login/list/select/fetch tests
- Register smtp-proxy project in playwright.config.ts
- Wait for TLS proxy readiness in docker-entrypoint.sh

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: enforce auth over TLS when STARTTLS is configured

- Set auth_require_tls conditionally based on tls_context presence
- Disable insecure SSLv2/SSLv3 protocols in TLS context

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: replace cert-gen service with inline cert generation

The cert-gen one-shot container was exiting immediately after
generating certificates, triggering --abort-on-container-exit
and stopping all services before tests could run.

Replace with an entrypoint script in smtp-proxy-tls that generates
the self-signed cert before starting the proxy server.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

e2e/scripts/smtp-tls-entrypoint.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+CERT_DIR="/certs"
+mkdir -p "$CERT_DIR"
+
+if [ ! -f "$CERT_DIR/cert.pem" ] || [ ! -f "$CERT_DIR/key.pem" ]; then
+  echo "==> Generating self-signed TLS certificate"
+  openssl req -x509 -newkey rsa:2048 -nodes \
+    -keyout "$CERT_DIR/key.pem" -out "$CERT_DIR/cert.pem" \
+    -days 1 -subj "/CN=smtp-proxy-tls"
+  echo "    Certificate generated"
+fi
+
+exec python3 main.py