Add SMTP security mode selector with SSL/STARTTLS/none options and automatic port switching

Replaced boolean `smtp_use_tls` with explicit `smtp_security` enum field supporting "none", "starttls", and "ssl" modes. Added database migration to create `smtp_security` column with "starttls" default. Updated mailer to use SMTP_SSL client for direct SSL connections on port 465 and SMTP with STARTTLS for port 587. Modified admin settings UI to show dropdown selector with encryption options and auto-adjust

backend/app/mailer.py

@@ -23,9 +23,13 @@ def send_html_email(settings: AppSettings, recipient: str, subject: str, body_ht
     message.add_alternative(body_html, subtype="html")
 
     context = ssl.create_default_context()
-    with smtplib.SMTP(settings.smtp_host, settings.smtp_port or 587, timeout=20) as server:
+    security = getattr(settings, "smtp_security", "") or ("starttls" if settings.smtp_use_tls else "none")
+    port = settings.smtp_port or (465 if security == "ssl" else 587)
+    smtp_client = smtplib.SMTP_SSL if security == "ssl" else smtplib.SMTP
+    connection = smtp_client(settings.smtp_host, port, timeout=20, context=context) if security == "ssl" else smtp_client(settings.smtp_host, port, timeout=20)
+    with connection as server:
         server.ehlo()
-        if settings.smtp_use_tls:
+        if security == "starttls":
             server.starttls(context=context)
             server.ehlo()
         if settings.smtp_username: