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>

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