Add LiteLLM proxy support for ChatGPT subscription AI access

- Add ai_provider setting: 'openai' (API key) or 'litellm' (ChatGPT subscription proxy)
- Auto-strip max_tokens/max_completion_tokens for chatgpt/ prefix models
  (ChatGPT subscription backend rejects token limit fields)
- LiteLLM mode: dummy API key when none configured, base URL required
- isOpenAIConfigured() checks base URL instead of API key for LiteLLM
- listAvailableModels() returns manualEntry flag for LiteLLM (no models.list)
- Settings UI: conditional fields, info banner, manual model input with
  chatgpt/ prefix examples when LiteLLM selected
- All 7 AI services work transparently via buildCompletionParams()

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

src/server/routers/settings.ts

@@ -201,8 +201,8 @@ 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')) {
+      // Reset OpenAI client if API key, base URL, model, or provider changed
+      if (input.settings.some((s) => s.key === 'openai_api_key' || s.key === 'openai_base_url' || s.key === 'ai_model' || s.key === 'ai_provider')) {
         const { resetOpenAIClient } = await import('@/lib/openai')
         resetOpenAIClient()
       }
@@ -247,6 +247,15 @@ export const settingsRouter = router({
   listAIModels: superAdminProcedure.query(async () => {
     const result = await listAvailableModels()
 
+    // LiteLLM mode: manual model entry, no listing available
+    if (result.manualEntry) {
+      return {
+        success: true,
+        models: [],
+        manualEntry: true,
+      }
+    }
+
     if (!result.success || !result.models) {
       return {
         success: false,