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/ai-settings-form.tsx

@@ -36,6 +36,7 @@ const formSchema = z.object({
   ai_model: z.string(),
   ai_send_descriptions: z.boolean(),
   openai_api_key: z.string().optional(),
+  openai_base_url: z.string().optional(),
 })
 
 type FormValues = z.infer<typeof formSchema>
@@ -47,6 +48,7 @@ interface AISettingsFormProps {
     ai_model?: string
     ai_send_descriptions?: string
     openai_api_key?: string
+    openai_base_url?: string
   }
 }
 
@@ -61,6 +63,7 @@ export function AISettingsForm({ settings }: AISettingsFormProps) {
       ai_model: settings.ai_model || 'gpt-4o',
       ai_send_descriptions: settings.ai_send_descriptions === 'true',
       openai_api_key: '',
+      openai_base_url: settings.openai_base_url || '',
     },
   })
 
@@ -113,6 +116,9 @@ export function AISettingsForm({ settings }: AISettingsFormProps) {
       settingsToUpdate.push({ key: 'openai_api_key', value: data.openai_api_key })
     }
 
+    // Save base URL (empty string clears it)
+    settingsToUpdate.push({ key: 'openai_base_url', value: data.openai_base_url?.trim() || '' })
+
     updateSettings.mutate({ settings: settingsToUpdate })
   }
 
@@ -208,6 +214,27 @@ export function AISettingsForm({ settings }: AISettingsFormProps) {
           )}
         />
 
+        <FormField
+          control={form.control}
+          name="openai_base_url"
+          render={({ field }) => (
+            <FormItem>
+              <FormLabel>API Base URL (Optional)</FormLabel>
+              <FormControl>
+                <Input
+                  placeholder="https://api.openai.com/v1"
+                  {...field}
+                />
+              </FormControl>
+              <FormDescription>
+                Custom base URL for OpenAI-compatible providers. Leave blank for OpenAI.
+                Use <code className="text-xs bg-muted px-1 rounded">https://openrouter.ai/api/v1</code> for OpenRouter (access Claude, Gemini, Llama, etc.)
+              </FormDescription>
+              <FormMessage />
+            </FormItem>
+          )}
+        />
+
         <FormField
           control={form.control}
           name="ai_model"