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/services/extension.service.js

@@ -42,7 +42,7 @@ const getExtensionTimeoutAction = async () => {
  *   - belong to this customer
  *   - be in `confirmed` status (not yet consumed)
  *   - have `is_extension = true`
- *   - have `is_free_trial = false` (extensions never use free trial)
+ *   - have `is_first_session_discount = false` (extensions never use the first-session discount)
  *
  * The payment session is NOT consumed at request time. It is consumed at approval moment
  * (mitra explicit accept OR auto-approve fires).
@@ -83,9 +83,9 @@ export const requestExtension = async (sessionId, customerId, { duration_minutes
       code: 'INVALID_STATE', statusCode: 409,
     })
   }
-  if (paySession.is_free_trial) {
-    throw Object.assign(new Error('Free trial is not available for extensions'), {
-      code: 'FREE_TRIAL_NOT_ALLOWED', statusCode: 400,
+  if (paySession.is_first_session_discount) {
+    throw Object.assign(new Error('First-session discount is not available for extensions'), {
+      code: 'FIRST_SESSION_DISCOUNT_NOT_ALLOWED', statusCode: 400,
     })
   }