Tianji custom event tracking (auth, billing, chat) (#36) (#45)

## Summary

Closes #36.

Helpers / consent gating already exist from [#42](#42). This PR adds **call sites** + docs:

- `identifyAccount` + `AnalyticsSession` (identify on session; `Logout` when auth ends)
- Auth: Sign In / Sign Up / AuthCallback (SSO) success & fail
- Billing: Checkout started, Payment Success / Canceled
- Chat: Conversation Created, Message Sent (metadata only — no prompt text)
- ToS Acknowledged
- `BILLING_PORTAL_OPENED` reserved until Account portal (#33)
- Catalog: [`llm-fe/ANALYTICS.md`](llm-fe/ANALYTICS.md)

No `chat_backend` changes required.

## Test plan

- [ ] Grant consent on beta → sign in → Tianji shows Login Success + identify
- [ ] Decline consent → same actions produce no custom events (page views still ok)
- [ ] Sign up + checkout + cancel/success return URLs fire expected events
- [ ] New chat + send message → Conversation Created / Message Sent without prompt text
- [ ] Acknowledge ToS → ToS Acknowledged
- [ ] `npm run test:ci -- --testPathPattern='analytics.test|SignIn.test|SignUp.test|AuthCallback.test'`Reviewed-on: #45

llm-fe/src/llm-fe/utils/analytics.ts

@@ -1,4 +1,5 @@
 import { hasAnalyticsConsent, isAnalyticsEnvironment } from './analyticsConsent';
+import type { Account } from '../data';
 
 type QueuedCall =
   | { type: 'track'; eventName: string; data?: Record<string, unknown> }
@@ -6,7 +7,21 @@ type QueuedCall =
 
 const queue: QueuedCall[] = [];
 
-/** Named events for product analytics. Page views stay on automatic tracker.js. */
+/**
+ * Named product events for Tianji (consent-gated). Page views stay on tracker.js.
+ *
+ * | Event | When |
+ * |-------|------|
+ * | Login Success / Failed | Password or SSO sign-in |
+ * | Sign Up Started / Success / Failed | Self-serve registration |
+ * | Logout | Sign out |
+ * | Checkout Started | Stripe Checkout session created |
+ * | Payment Success / Canceled | Billing return URLs |
+ * | Conversation Created | New chat id assigned over WS |
+ * | Message Sent | User submits prompt (no content) |
+ * | ToS Acknowledged | POST acknowledge_tos succeeds |
+ * | Billing Portal Opened | When #33 portal CTA ships |
+ */
 export const AnalyticsEvents = {
   LOGIN_SUCCESS: 'Login Success',
   LOGIN_FAILED: 'Login Failed',
@@ -75,3 +90,17 @@ export const identifyUser = (userInfo: Record<string, unknown>): void => {
 
   queue.push({ type: 'identify', userInfo: { ...userInfo } });
 };
+
+/** Identify with stable non-sensitive traits only (no names / prompts). */
+export const identifyAccount = (account: Account): void => {
+  const traits: Record<string, unknown> = {
+    userId: account.email,
+  };
+  if (account.company?.id != null) {
+    traits.companyId = account.company.id;
+  }
+  if (account.is_company_manager) {
+    traits.isCompanyManager = true;
+  }
+  identifyUser(traits);
+};