feat: register Freebuff provider in all index files

- Add Freebuff to providers registry (open-sse/providers/registry/index.js)
- Register FreebuffExecutor (open-sse/executors/index.js)
- Add Freebuff OAuth config (src/lib/oauth/providers/index.js)
- Register Freebuff usage handler (open-sse/services/usage.js)

This enables Freebuff provider to appear in dashboard and be fully functional.

open-sse/services/usage.js

@@ -14,6 +14,7 @@ import { getCodeBuddyCnUsage, getCodeBuddyIntlUsage } from "./usage/codebuddy-cn
 import { getGrokCliUsage } from "./usage/grok-cli.js";
 import { getKimiUsage } from "./usage/kimi.js";
 import { getDeepseekUsage } from "./usage/deepseek.js";
+import { getFreebuffUsage } from "./usage/freebuff.js";
 import { resolveQoderCredentials } from "./qoderModels.js";
 import {
   getIflowUsage,
@@ -54,6 +55,7 @@ const USAGE_HANDLERS = {
   "grok-cli": (c) => getGrokCliUsage(c.accessToken, c.providerSpecificData, c.proxyOptions),
   kimi: (c) => getKimiUsage(c.accessToken, c.apiKey, c.proxyOptions, c.providerSpecificData),
   deepseek: (c) => getDeepseekUsage(c.apiKey, c.proxyOptions),
+  freebuff: (c) => getFreebuffUsage(c.accessToken, c.providerSpecificData, c.proxyOptions),
 };
 
 export async function getUsageForProvider(connection, proxyOptions = null) {