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/app/api/billing/create-pack-checkout/route.ts

@@ -0,0 +1,135 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { auth } from '@/auth'
+import { stripe } from '@/lib/stripe'
+import { isBillingEnabled } from '@/lib/billing/stripe-prices'
+import {
+  isCreditPackId,
+  resolvePackPriceId,
+  getCreditPack,
+} from '@/lib/billing/credit-packs'
+import { prisma } from '@/lib/prisma'
+import { z } from 'zod'
+
+const bodySchema = z.object({
+  packId: z.enum(['S', 'M', 'L']),
+})
+
+export async function POST(req: NextRequest) {
+  const session = await auth()
+  if (!session?.user?.id || !session.user.email) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+  }
+
+  if (!(await isBillingEnabled())) {
+    return NextResponse.json({ error: 'Billing is not enabled' }, { status: 403 })
+  }
+
+  const secret = process.env.STRIPE_SECRET_KEY
+  if (!secret || secret === 'sk_test_placeholder') {
+    return NextResponse.json(
+      { error: 'Stripe is not configured (STRIPE_SECRET_KEY missing)' },
+      { status: 503 },
+    )
+  }
+
+  const parsed = bodySchema.safeParse(await req.json())
+  if (!parsed.success || !isCreditPackId(parsed.data.packId)) {
+    return NextResponse.json({ error: 'Invalid pack' }, { status: 400 })
+  }
+
+  const packId = parsed.data.packId
+  const pack = getCreditPack(packId)
+  const userId = session.user.id
+  const userEmail = session.user.email
+
+  try {
+    let priceId: string
+    try {
+      priceId = await resolvePackPriceId(packId)
+    } catch (e) {
+      console.error('[billing/create-pack-checkout] price resolve failed:', e)
+      return NextResponse.json(
+        { error: 'Credit pack price IDs not configured. Set them in Admin > Billing.' },
+        { status: 503 },
+      )
+    }
+
+    if (priceId.startsWith('price_mock_')) {
+      return NextResponse.json(
+        { error: 'Credit pack price IDs not configured. Set them in Admin > Billing.' },
+        { status: 503 },
+      )
+    }
+
+    const subscription = await prisma.subscription.findUnique({ where: { userId } })
+    let customerId = subscription?.stripeCustomerId ?? undefined
+
+    if (customerId && customerId.startsWith('cus_mock')) {
+      customerId = undefined
+    }
+
+    if (!customerId) {
+      const customer = await stripe.customers.create({
+        email: userEmail,
+        metadata: { userId },
+      })
+      customerId = customer.id
+
+      await prisma.subscription.upsert({
+        where: { userId },
+        update: { stripeCustomerId: customerId },
+        create: {
+          userId,
+          stripeCustomerId: customerId,
+          tier: 'BASIC',
+          status: 'ACTIVE',
+          currentPeriodStart: new Date(),
+          currentPeriodEnd: new Date(Date.now() + 30 * 24 * 3600 * 1000),
+        },
+      })
+    }
+
+    const host = req.headers.get('x-forwarded-host') ?? req.headers.get('host') ?? 'localhost:3000'
+    const proto = req.headers.get('x-forwarded-proto') ?? 'http'
+    const origin = `${proto}://${host}`
+
+    const checkoutSession = await stripe.checkout.sessions.create({
+      customer: customerId,
+      mode: 'payment',
+      line_items: [{ price: priceId, quantity: 1 }],
+      success_url: `${origin}/settings/billing?session_id={CHECKOUT_SESSION_ID}&pack=1`,
+      cancel_url: `${origin}/settings/billing?canceled=1`,
+      metadata: {
+        userId,
+        type: 'credit_pack',
+        packId,
+        credits: String(pack.credits),
+      },
+      payment_intent_data: {
+        metadata: {
+          userId,
+          type: 'credit_pack',
+          packId,
+          credits: String(pack.credits),
+        },
+      },
+      customer_update: { address: 'auto' },
+      allow_promotion_codes: true,
+    })
+
+    if (!checkoutSession.url) {
+      return NextResponse.json({ error: 'Checkout session has no URL' }, { status: 500 })
+    }
+
+    return NextResponse.json({
+      url: checkoutSession.url,
+      sessionId: checkoutSession.id,
+      packId,
+      credits: pack.credits,
+    })
+  } catch (error) {
+    console.error('[billing/create-pack-checkout]', error)
+    const msg = error instanceof Error ? error.message : 'Failed to create pack checkout'
+    return NextResponse.json({ error: msg }, { status: 500 })
+  }
+}