Pricing: migrate from app_config JSON to relational tables

Replaces the two `pricing_*_tiers_json` blobs and five `first_session_discount_*`
keys in app_config with dedicated `pricing_tiers` and `pricing_promotions`
tables plus matching `_history` audit tables. UUID PKs, UNIQUE(mode, minutes)
natural-key constraint, optimistic-lock via `updated_at` token returning 409
STALE_WRITE on conflicts. Every mutation writes a history row capturing the
operator (changed_by from request.auth.userId) and change_kind.

CC SettingsPage replaces the JSON-textarea editors with per-row tables —
add / edit / soft-delete / reactivate / reorder, plus a buffered first-session
discount form with the same optimistic-lock contract. `minutes` and `mode` are
read-only on edit since they form the natural key; operators soft-delete and
recreate to change duration.

Stage 5 fixes a latent leak: `client.payment.routes.js` had its own local
`readDiscountConfig` that still read from app_config — would have silently
fallen to hardcoded defaults once the legacy rows were deleted. Now reads from
pricing_promotions via the shared service helper, so CC edits to the first-
session discount affect actual payment pricing on the next request.

Customer-facing GET /api/client/chat/pricing shape unchanged (id values are
now UUIDs instead of "5"/"12"/"60" but lookups happen by (mode, minutes), so
no app changes needed). 27 new backend tests, all green.

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

backend/test/helpers/fixtures.js

@@ -57,6 +57,44 @@ export const createMitra = async ({
  */
 export const seedDefaultConfig = () => resetAppConfig()
 
+/**
+ * Insert (or fetch) a control-center user with full `config` permissions. Used by
+ * /internal/config/* route tests that need a JWT subject that survives the
+ * `attachCcUser` + `requirePermission('config', …)` preHandler chain.
+ *
+ * Idempotent: re-runs return the same row by email. We do NOT truncate cc_user / roles
+ * between tests (db.js documents the rationale), so subsequent test files inherit
+ * whatever this seeded.
+ */
+export const createCcUser = async ({
+  email = `cc-test-${randomUUID().slice(0, 8)}@halobestie.test`,
+  displayName = 'CC Test User',
+  permissions = {
+    mitra: ['create', 'read', 'update', 'delete'],
+    control_center_users: ['create', 'read', 'update', 'delete'],
+    config: ['read', 'update'],
+    roles: ['create', 'read', 'update', 'delete'],
+  },
+} = {}) => {
+  const sql = db()
+  // One role per test invocation, named after a slice of the email so re-runs don't
+  // collide with the seeded `super_admin` role from seed.js.
+  const roleName = `cc-test-role-${email.slice(0, 16)}`
+  const [role] = await sql`
+    INSERT INTO roles (name, permissions)
+    VALUES (${roleName}, ${sql.json(permissions)})
+    ON CONFLICT (name) DO UPDATE SET permissions = EXCLUDED.permissions
+    RETURNING id
+  `
+  const [user] = await sql`
+    INSERT INTO control_center_users (email, display_name, role_id, password_hash)
+    VALUES (${email}, ${displayName}, ${role.id}, 'unused-for-jwt-tests')
+    ON CONFLICT (email) DO UPDATE SET role_id = EXCLUDED.role_id
+    RETURNING id, email, display_name, role_id, created_at
+  `
+  return { ...user, role: { id: role.id, permissions } }
+}
+
 /**
  * Convenience: full reset between tests. Truncates Phase 3.7 tables, restores
  * default config rows.