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/test/services/payment.service.test.js

@@ -38,8 +38,9 @@ describe('payment.service', () => {
     expect(session.customer_id).toBe(customer.id)
     expect(session.duration_minutes).toBe(15)
     expect(session.amount).toBe(30000)
-    expect(session.is_free_trial).toBe(false)
+    expect(session.is_first_session_discount).toBe(false)
     expect(session.is_extension).toBe(false)
+    expect(session.mode).toBe('chat')
     expect(new Date(session.expires_at).getTime()).toBeGreaterThan(before)
 
     // Verify it's actually persisted (not just returned from the INSERT)