refactor(quota): modularize quota providers and add docs map (#427)

* refactor(quota): migrate server quota providers to modular registry architecture

* docs: add quota module documentation

Explain the quota module purpose and runtime signals.
List entrypoints, provider registry, and response shape.
Outline steps to add a new provider and testing commands.

packages/web/server/lib/quota/index.js

@@ -0,0 +1,21 @@
+/**
+ * Quota module
+ *
+ * Provides quota usage tracking for various AI provider services.
+ * @module quota
+ */
+
+export {
+  listConfiguredQuotaProviders,
+  fetchQuotaForProvider,
+  fetchClaudeQuota,
+  fetchOpenaiQuota,
+  fetchGoogleQuota,
+  fetchCodexQuota,
+  fetchCopilotQuota,
+  fetchCopilotAddonQuota,
+  fetchKimiQuota,
+  fetchOpenRouterQuota,
+  fetchZaiQuota,
+  fetchNanoGptQuota
+} from './providers/index.js';