feat: seed FirebaseServiceAccountJson plugin configuration value

Adds an empty default so the key exists in PluginConfigurationValues
on first plugin install. Admins only need to paste the Firebase service
account JSON into the database to enable push notifications.

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

Author: renemadsen

eFormAPI/Plugins/TimePlanning.Pn/TimePlanning.Pn/Infrastructure/Data/Seed/Data/TimePlanningConfigurationSeedData.cs

@@ -216,6 +216,11 @@ public class TimePlanningConfigurationSeedData : IPluginConfigurationSeedData
         {
             Name = $"{TimePlanningBaseSettingsName}:SnapshotEnabled",
             Value = "0"
+        },
+        new PluginConfigurationValue
+        {
+            Name = $"{TimePlanningBaseSettingsName}:FirebaseServiceAccountJson",
+            Value = ""
         }
     ];
 }