fix: Increase openAiPrompt max size from 6KB to 1MB

Allow larger custom system prompts for AI processing to accommodate
more complex instructions and context.

Author: andris9

lib/schemas.js

@@ -174,7 +174,7 @@ const settingsSchema = {
 
     openAiPrompt: Joi.string()
         .allow('')
-        .max(6 * 1024)
+        .max(1024 * 1024)
         .example('You are an assistant scanning incoming emails…')
         .description('Custom system prompt to guide AI behavior when processing emails')
         .label('OpenAiPrompt'),