fix(api): avoid NRE in MailSender on rdv query notifications

The API did not bind the Smtp section and its appsettings lacked
Site.Owner, so MailSender threw a NullReferenceException and
PostQuery returned 500. Bind Smtp in the API, add Owner/Smtp to the
api appsettings template, and log/skip sending when mail
configuration is incomplete.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

contrib/template.appsettings-api.json

@@ -6,7 +6,17 @@
       "https://preprod-blogs.$DOMAIN"
     ],
     "Bills": "bills",
-    "TempDir": "temp"
+    "TempDir": "temp",
+    "Owner": {
+      "Name": "Paul Schneider",
+      "EMail": "paul@pschneider.fr"
+    }
+  },
+  "Smtp": {
+    "Host": "mercure.pschneider.fr",
+    "Port": 465,
+    "UserName": "paul",
+    "Password": "$SMTP_PASSWORD"
   },
   "Logging": {
     "LogLevel": {