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/lib/openai.ts

@@ -8,6 +8,33 @@ const globalForOpenAI = globalThis as unknown as {
   openaiInitialized: boolean
 }
 
+// ─── Provider Detection ─────────────────────────────────────────────────────
+
+/**
+ * Get the configured AI provider from SystemSettings.
+ * Returns 'openai' (default) or 'litellm' (ChatGPT subscription proxy).
+ */
+export async function getConfiguredProvider(): Promise<'openai' | 'litellm'> {
+  try {
+    const setting = await prisma.systemSettings.findUnique({
+      where: { key: 'ai_provider' },
+    })
+    const value = setting?.value || 'openai'
+    return value === 'litellm' ? 'litellm' : 'openai'
+  } catch {
+    return 'openai'
+  }
+}
+
+/**
+ * Check if a model ID indicates LiteLLM ChatGPT subscription routing.
+ * Models like 'chatgpt/gpt-5.2' use the chatgpt/ prefix.
+ * Used by buildCompletionParams (sync) to strip unsupported token limit fields.
+ */
+export function isLiteLLMChatGPTModel(model: string): boolean {
+  return model.toLowerCase().startsWith('chatgpt/')
+}
+
 // ─── Model Type Detection ────────────────────────────────────────────────────
 
 /**
@@ -168,6 +195,12 @@ export function buildCompletionParams(
     params.response_format = { type: 'json_object' }
   }
 
+  // LiteLLM ChatGPT subscription models reject token limit fields
+  if (isLiteLLMChatGPTModel(model)) {
+    delete params.max_tokens
+    delete params.max_completion_tokens
+  }
+
   return params
 }
 
@@ -209,8 +242,12 @@ async function getBaseURL(): Promise<string | undefined> {
  */
 async function createOpenAIClient(): Promise<OpenAI | null> {
   const apiKey = await getOpenAIApiKey()
+  const provider = await getConfiguredProvider()
 
-  if (!apiKey) {
+  // LiteLLM proxy may not require a real API key
+  const effectiveApiKey = apiKey || (provider === 'litellm' ? 'sk-litellm' : null)
+
+  if (!effectiveApiKey) {
     console.warn('OpenAI API key not configured')
     return null
   }
@@ -218,11 +255,11 @@ async function createOpenAIClient(): Promise<OpenAI | null> {
   const baseURL = await getBaseURL()
 
   if (baseURL) {
-    console.log(`[OpenAI] Using custom base URL: ${baseURL}`)
+    console.log(`[OpenAI] Using custom base URL: ${baseURL} (provider: ${provider})`)
   }
 
   return new OpenAI({
-    apiKey,
+    apiKey: effectiveApiKey,
     ...(baseURL ? { baseURL } : {}),
   })
 }
@@ -259,6 +296,12 @@ export function resetOpenAIClient(): void {
  * Check if OpenAI is configured and available
  */
 export async function isOpenAIConfigured(): Promise<boolean> {
+  const provider = await getConfiguredProvider()
+  if (provider === 'litellm') {
+    // LiteLLM just needs a base URL configured
+    const baseURL = await getBaseURL()
+    return !!baseURL
+  }
   const apiKey = await getOpenAIApiKey()
   return !!apiKey
 }
@@ -270,8 +313,20 @@ export async function listAvailableModels(): Promise<{
   success: boolean
   models?: string[]
   error?: string
+  manualEntry?: boolean
 }> {
   try {
+    const provider = await getConfiguredProvider()
+
+    // LiteLLM proxy for ChatGPT subscription doesn't support models.list()
+    if (provider === 'litellm') {
+      return {
+        success: true,
+        models: [],
+        manualEntry: true,
+      }
+    }
+
     const client = await getOpenAI()
 
     if (!client) {