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/routes/admin.py

@@ -163,6 +163,7 @@ def read_smtp_settings(req: Request, db: Session = Depends(get_db)):
         "smtp_password_configured": bool(settings.smtp_password),
         "smtp_from_email": settings.smtp_from_email,
         "smtp_from_name": settings.smtp_from_name,
+        "smtp_security": getattr(settings, "smtp_security", "starttls") or ("starttls" if settings.smtp_use_tls else "none"),
         "smtp_use_tls": settings.smtp_use_tls,
         "app_base_url": settings.app_base_url,
     }
@@ -181,7 +182,11 @@ def update_smtp_settings(req: Request, data: dict, db: Session = Depends(get_db)
     settings.smtp_username = (data.get("smtp_username") or "").strip()
     settings.smtp_from_email = (data.get("smtp_from_email") or "").strip()
     settings.smtp_from_name = (data.get("smtp_from_name") or "Cluedo HP").strip()
-    settings.smtp_use_tls = bool(data.get("smtp_use_tls", True))
+    security = data.get("smtp_security") or ("starttls" if data.get("smtp_use_tls", True) else "none")
+    if security not in ("none", "starttls", "ssl"):
+        raise HTTPException(400, "invalid SMTP security mode")
+    settings.smtp_security = security
+    settings.smtp_use_tls = security == "starttls"
     settings.app_base_url = (data.get("app_base_url") or "http://localhost:8081").strip().rstrip("/")
     if "smtp_password" in data and data.get("smtp_password"):
         settings.smtp_password = data["smtp_password"]