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/helpers/db.js

@@ -52,7 +52,8 @@ export const resetDbHard = async () => {
 
 /**
  * Drop and re-seed the configurable app_config rows back to their canonical defaults.
- * Tests that mutate config (e.g. flipping free_trial_enabled) call this in afterEach.
+ * Tests that mutate config (e.g. flipping first_session_discount_enabled) call this
+ * in afterEach.
  */
 export const resetAppConfig = async () => {
   const sql = db()
@@ -61,8 +62,6 @@ export const resetAppConfig = async () => {
   const defaults = [
     ['anonymity', { enabled: false }],
     ['max_customers_per_mitra', { value: 3 }],
-    ['free_trial_enabled', { value: true }],
-    ['free_trial_duration_minutes', { value: 5 }],
     ['extension_timeout_seconds', { value: 60 }],
     ['early_end_mitra_enabled', { value: false }],
     ['early_end_customer_enabled', { value: false }],
@@ -70,6 +69,13 @@ export const resetAppConfig = async () => {
     ['returning_chat_confirmation_timeout_seconds', { value: 20 }],
     ['extension_default_action_on_timeout', { value: 'auto_approve' }],
     ['pairing_blast_timeout_seconds', { value: 60 }],
+    // Phase 4
+    ['first_session_discount_enabled', { value: true }],
+    ['first_session_discount_actual_price_idr', { value: 2000 }],
+    ['first_session_discount_gimmick_price_idr', { value: 12000 }],
+    ['first_session_discount_duration_minutes', { value: 12 }],
+    ['first_session_discount_modes', { value: ['chat'] }],
+    ['three_minute_warning_enabled', { value: true }],
   ]
   for (const [key, value] of defaults) {
     await sql`