Reduce AI costs: switch tagging to gpt-4o-mini, add custom base URL support

- Change AI tagging to use AI_MODELS.QUICK (gpt-4o-mini) instead of gpt-4o for
  10-15x cost reduction on classification tasks
- Add openai_base_url system setting for OpenAI-compatible providers
  (OpenRouter, Groq, Together AI, local models)
- Reset OpenAI client singleton when API key, base URL, or model changes
- Add base URL field to AI settings form with provider examples

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

src/components/settings/settings-content.tsx

@@ -84,6 +84,7 @@ export function SettingsContent({ initialSettings, isSuperAdmin = true }: Settin
     'ai_model',
     'ai_send_descriptions',
     'openai_api_key',
+    'openai_base_url',
   ])
 
   const brandingSettings = getSettingsByKeys([