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/actions/admin-settings.ts

@@ -42,24 +42,47 @@ export async function getSystemConfig() {
   return getCachedConfig()
 }
 
+/** Keys that may be cleared (empty string) to fall back to chat / defaults. */
+const CLEARABLE_CONFIG_KEYS = new Set([
+  'AI_PROVIDER_SLIDES',
+  'AI_MODEL_SLIDES',
+])
+
 export async function updateSystemConfig(data: Record<string, string>) {
   await checkAdmin()
 
   try {
-    // Filter out empty values but keep 'false' as valid
-    const filteredData = Object.fromEntries(
-      Object.entries(data).filter(([key, value]) => value !== '' && value !== null && value !== undefined)
-    )
+    // Filter out empty values but keep 'false' as valid.
+    // Slides provider/model may be empty intentionally (= use Chat fallback).
+    const toUpsert: Record<string, string> = {}
+    const toDelete: string[] = []
 
-    const operations = Object.entries(filteredData).map(([key, value]) =>
-      prisma.systemConfig.upsert({
-        where: { key },
-        update: { value },
-        create: { key, value }
-      })
-    )
+    for (const [key, value] of Object.entries(data)) {
+      if (value === null || value === undefined) continue
+      if (value === '' && CLEARABLE_CONFIG_KEYS.has(key)) {
+        toDelete.push(key)
+        continue
+      }
+      if (value === '') continue
+      toUpsert[key] = value
+    }
 
-    await prisma.$transaction(operations)
+    const operations = [
+      ...Object.entries(toUpsert).map(([key, value]) =>
+        prisma.systemConfig.upsert({
+          where: { key },
+          update: { value },
+          create: { key, value },
+        }),
+      ),
+      ...toDelete.map((key) =>
+        prisma.systemConfig.deleteMany({ where: { key } }),
+      ),
+    ]
+
+    if (operations.length > 0) {
+      await prisma.$transaction(operations)
+    }
 
     return { success: true }
   } catch (error) {