feat: résumé progressif chat — compression automatique du contexte

- Migration: champs summary + summaryUpTo sur Conversation
- Compression tous les 8 messages (garde les 4 derniers intacts)
- Résumé régénéré tous les 4 nouveaux messages
- Fallback gracieux: si la génération échoue, envoie tout le contexte
- getChatProvider import ajouté
- i18n non requis (optimisation backend)

memento-note/prisma/schema.prisma

@@ -456,15 +456,17 @@ model AgentAction {
 }
 
 model Conversation {
-  id         String        @id @default(cuid())
-  title      String?
-  userId     String
-  notebookId String?
-  createdAt  DateTime      @default(now())
-  updatedAt  DateTime      @updatedAt
-  messages   ChatMessage[]
-  notebook   Notebook?     @relation(fields: [notebookId], references: [id])
-  user       User          @relation(fields: [userId], references: [id], onDelete: Cascade)
+  id              String        @id @default(cuid())
+  title           String?
+  userId          String
+  notebookId      String?
+  summary         String?
+  summaryUpTo     String?
+  createdAt       DateTime      @default(now())
+  updatedAt       DateTime      @updatedAt
+  messages        ChatMessage[]
+  notebook        Notebook?     @relation(fields: [notebookId], references: [id])
+  user            User          @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([userId])
   @@index([notebookId])