Phase 4 Stage 1: backend foundation (additive endpoints + schema)

Schema (idempotent migration):
- payment_sessions.is_free_trial -> is_first_session_discount (data copied)
- payment_sessions.mode TEXT NOT NULL DEFAULT 'chat' CHECK (chat|call)
- chat_sessions.topics TEXT[] for ESP picks (info-only)

New endpoints:
- GET /api/client/onboarding-state (drives verif sheet + S6 paywall gate)
- GET /api/client/chat-pricing (rewrite: chat+call groups + first-session
  discount block, per-customer eligibility)
- GET /api/shared/auth-providers (env-probed; replaces ENABLE_SOCIAL_AUTH
  build flag — frontend cutover lands in stage 2)
- GET /api/client/support-handles (Tanya Admin handles, CC-config-driven)

session_warning WS event fires once at 180s remaining.

app_config seeds (mock pricing tiers, first-session discount, support
handles, payment method order, end-session 2-step toggle).

CC SettingsPage: 3 new sections (first-session discount, pricing tiers
JSON editors, support handles).

15/15 Vitest passing. chat_sessions.is_free_trial also renamed for
consistency (plan only specified payment_sessions; pairing.service.js
read both).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/routes/public/client.chat.routes.js

@@ -35,7 +35,10 @@ const resolveCustomer = async (request, reply) => {
 }
 
 export const clientChatRoutes = async (app) => {
-  // Get pricing tiers + free trial eligibility
+  // Get chat + call pricing tiers + first-session-discount eligibility (per-customer).
+  // Phase 4 reshape — tiers come from `app_config.pricing_{chat,call}_tiers_json` and
+  // discount eligibility is the AND of: phone-verified + no completed sessions +
+  // first_session_discount_enabled.
   app.get('/pricing', { preHandler: [authenticate, resolveCustomer] }, async (request, reply) => {
     const pricing = await getPricingForCustomer(request.customer.id)
     return reply.send({ success: true, data: pricing })
@@ -171,7 +174,7 @@ export const clientChatRoutes = async (app) => {
 
   /**
    * Extension request REQUIRES `extension_payment_session_id`.
-   * The payment session must be is_extension=true and is_free_trial=false.
+   * The payment session must be is_extension=true and is_first_session_discount=false.
    * Pricing/duration come from the payment session via the extension service.
    */
   app.post('/session/:sessionId/extend', { preHandler: [authenticate, resolveCustomer] }, async (request, reply) => {