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/lib/openai.ts

@@ -187,7 +187,25 @@ async function getOpenAIApiKey(): Promise<string | null> {
 }
 
 /**
- * Create OpenAI client instance
+ * Get custom base URL for OpenAI-compatible providers.
+ * Supports OpenRouter, Together AI, Groq, local models, etc.
+ * Set via Settings → AI or OPENAI_BASE_URL env var.
+ */
+async function getBaseURL(): Promise<string | undefined> {
+  try {
+    const setting = await prisma.systemSettings.findUnique({
+      where: { key: 'openai_base_url' },
+    })
+    return setting?.value || process.env.OPENAI_BASE_URL || undefined
+  } catch {
+    return process.env.OPENAI_BASE_URL || undefined
+  }
+}
+
+/**
+ * Create OpenAI client instance.
+ * Supports custom baseURL for OpenAI-compatible providers
+ * (OpenRouter, Groq, Together AI, local models, etc.)
  */
 async function createOpenAIClient(): Promise<OpenAI | null> {
   const apiKey = await getOpenAIApiKey()
@@ -197,8 +215,15 @@ async function createOpenAIClient(): Promise<OpenAI | null> {
     return null
   }
 
+  const baseURL = await getBaseURL()
+
+  if (baseURL) {
+    console.log(`[OpenAI] Using custom base URL: ${baseURL}`)
+  }
+
   return new OpenAI({
     apiKey,
+    ...(baseURL ? { baseURL } : {}),
   })
 }
 
@@ -221,6 +246,15 @@ export async function getOpenAI(): Promise<OpenAI | null> {
   return client
 }
 
+/**
+ * Reset the OpenAI client singleton (e.g., after settings change).
+ * Next call to getOpenAI() will create a fresh client.
+ */
+export function resetOpenAIClient(): void {
+  globalForOpenAI.openai = undefined
+  globalForOpenAI.openaiInitialized = false
+}
+
 /**
  * Check if OpenAI is configured and available
  */