feat(credits): solde IA unique (option M), packs Stripe et polish billing

Un pot de crédits global (BASIC 100 / PRO 1 000 / BUSINESS 4 000) remplace
les seaux par fonction côté débit. Packs one-shot S/M/L, admin sans seaux
legacy, usage multi-features, hints de coût, anti-flash thème et hydratation
admin. Inclut aussi le pipeline slides renforcé et la page publique polishée.

memento-note/lib/billing/credit-packs.ts

@@ -0,0 +1,158 @@
+/**
+ * Packs de crédits IA (achat one-shot Stripe, mode payment).
+ * Les crédits achetés s’ajoutent à `purchasedBalance` (reportables entre mois).
+ */
+
+import { getConfigValue } from '@/lib/config'
+import { stripe } from '@/lib/stripe'
+
+export type CreditPackId = 'S' | 'M' | 'L'
+
+export type CreditPackDef = {
+  id: CreditPackId
+  credits: number
+  /** Prix d’affichage par défaut (si Stripe non configuré) */
+  defaultDisplay: string
+  defaultAmount: number
+  currency: string
+  priceConfigKey: string
+}
+
+/** Catalogue fixe — crédits par pack (option M). */
+export const CREDIT_PACKS: Record<CreditPackId, CreditPackDef> = {
+  S: {
+    id: 'S',
+    credits: 100,
+    defaultDisplay: '4,90 €',
+    defaultAmount: 4.9,
+    currency: 'EUR',
+    priceConfigKey: 'STRIPE_PRICE_CREDITS_S',
+  },
+  M: {
+    id: 'M',
+    credits: 500,
+    defaultDisplay: '19,90 €',
+    defaultAmount: 19.9,
+    currency: 'EUR',
+    priceConfigKey: 'STRIPE_PRICE_CREDITS_M',
+  },
+  L: {
+    id: 'L',
+    credits: 2000,
+    defaultDisplay: '49,90 €',
+    defaultAmount: 49.9,
+    currency: 'EUR',
+    priceConfigKey: 'STRIPE_PRICE_CREDITS_L',
+  },
+}
+
+export const CREDIT_PACK_IDS = Object.keys(CREDIT_PACKS) as CreditPackId[]
+
+export function isCreditPackId(value: string): value is CreditPackId {
+  return value === 'S' || value === 'M' || value === 'L'
+}
+
+export function getCreditPack(id: CreditPackId): CreditPackDef {
+  return CREDIT_PACKS[id]
+}
+
+export async function resolvePackPriceId(packId: CreditPackId): Promise<string> {
+  const pack = CREDIT_PACKS[packId]
+  const fromDb = await getConfigValue(pack.priceConfigKey, '')
+  const priceId = fromDb || process.env[pack.priceConfigKey] || ''
+  if (priceId) return priceId
+
+  const isMock =
+    !process.env.STRIPE_SECRET_KEY || process.env.STRIPE_SECRET_KEY === 'sk_test_placeholder'
+  if (isMock && process.env.NODE_ENV !== 'test') {
+    return `price_mock_credits_${packId.toLowerCase()}`
+  }
+  throw new Error(`No Stripe price ID configured for credit pack ${packId}`)
+}
+
+export type PackPublicPrice = {
+  id: CreditPackId
+  credits: number
+  display: string
+  amount: number
+  currency: string
+  configured: boolean
+}
+
+function formatMoney(amount: number, currency: string): string {
+  const cur = currency.toUpperCase()
+  if (cur === 'EUR') {
+    return `${amount.toLocaleString('fr-FR', { minimumFractionDigits: 0, maximumFractionDigits: 2 })} €`
+  }
+  if (cur === 'USD') {
+    return `$${amount.toLocaleString('en-US', { minimumFractionDigits: 0, maximumFractionDigits: 2 })}`
+  }
+  if (cur === 'GBP') {
+    return `£${amount.toLocaleString('en-GB', { minimumFractionDigits: 0, maximumFractionDigits: 2 })}`
+  }
+  return `${amount} ${cur}`
+}
+
+export async function getPackPublicPrices(): Promise<PackPublicPrice[]> {
+  const isMock =
+    !process.env.STRIPE_SECRET_KEY || process.env.STRIPE_SECRET_KEY === 'sk_test_placeholder'
+
+  return Promise.all(
+    CREDIT_PACK_IDS.map(async (id) => {
+      const pack = CREDIT_PACKS[id]
+      let display = pack.defaultDisplay
+      let amount = pack.defaultAmount
+      let currency = pack.currency
+      let configured = false
+
+      try {
+        const priceId = await resolvePackPriceId(id)
+        if (priceId && !priceId.startsWith('price_mock_')) {
+          configured = true
+          if (!isMock) {
+            const price = await stripe.prices.retrieve(priceId)
+            if (price.unit_amount != null) {
+              amount = price.unit_amount / 100
+              currency = price.currency.toUpperCase()
+              display = formatMoney(amount, currency)
+            }
+          }
+        }
+      } catch {
+        /* pack non configuré — on garde le prix catalogue */
+      }
+
+      return {
+        id,
+        credits: pack.credits,
+        display,
+        amount,
+        currency,
+        configured: configured || isMock,
+      }
+    }),
+  )
+}
+
+/** Retrouve packId + crédits depuis un price ID Stripe (webhook). */
+export async function resolvePackFromPriceId(
+  priceId: string,
+): Promise<{ packId: CreditPackId; credits: number } | null> {
+  if (!priceId) return null
+  if (priceId.startsWith('price_mock_credits_')) {
+    const suffix = priceId.replace('price_mock_credits_', '').toUpperCase()
+    if (isCreditPackId(suffix)) {
+      return { packId: suffix, credits: CREDIT_PACKS[suffix].credits }
+    }
+  }
+
+  for (const id of CREDIT_PACK_IDS) {
+    const pack = CREDIT_PACKS[id]
+    const fromDb = await getConfigValue(pack.priceConfigKey, '')
+    const configured = fromDb || process.env[pack.priceConfigKey] || ''
+    if (configured && configured === priceId) {
+      return { packId: id, credits: pack.credits }
+    }
+  }
+  return null
+}