feat: add Gemini 2.5 Flash as default model option

Added gemini-2.5-flash to the admin panel selector (first/default option)
and updated the fallback default in AiBot and migrate seed.

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

setup/migrate.php

@@ -200,7 +200,7 @@ $seedSettings = [
     'openai_api_key'                   => '',
     'openai_model'                     => 'gpt-4o-mini',
     'gemini_api_key'                   => '',
-    'gemini_model'                     => 'gemini-2.0-flash',
+    'gemini_model'                     => 'gemini-2.5-flash',
     'ai_max_tokens'                    => '500',
     'ai_default_prompt'                => 'Eres un asistente virtual de Palmas360. Responde de forma amable y profesional.',
 ];