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/server/routers/settings.ts

@@ -201,6 +201,12 @@ export const settingsRouter = router({
         clearStorageProviderCache()
       }
 
+      // Reset OpenAI client if API key or base URL changed
+      if (input.settings.some((s) => s.key === 'openai_api_key' || s.key === 'openai_base_url' || s.key === 'ai_model')) {
+        const { resetOpenAIClient } = await import('@/lib/openai')
+        resetOpenAIClient()
+      }
+
       // Audit log
       await logAudit({
         prisma: ctx.prisma,