Add ping_threshold to mailer config to prevent SES SMTP timeout errors

Set ping_threshold to 10 seconds so Symfony checks and reconnects stale
SMTP connections before reuse, fixing "451 4.4.2 Timeout waiting for
data from client" errors from Amazon SES.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/packages/mailer.yaml

@@ -1,3 +1,4 @@
 framework:
     mailer:
         dsn: '%env(MAILER_DSN)%'
+        ping_threshold: 10