Implement incremental sync and offline recovery

backend/prisma/schema.prisma

@@ -12,6 +12,7 @@ model User {
   slug           String          @unique
   displayName    String          @map("display_name")
   isDefault      Boolean         @default(false) @map("is_default")
+  libraryCursor  BigInt          @default(0) @map("library_cursor")
   createdAt      DateTime        @default(now()) @map("created_at")
   updatedAt      DateTime        @updatedAt @map("updated_at")
   devices        Device[]
@@ -20,6 +21,7 @@ model User {
   artworkAssets  ArtworkAsset[]
   uploadSessions UploadSession[]
   libraryEvents  LibraryEvent[]
+  syncCursors    DeviceSyncCursor[]
 
   @@map("users")
 }
@@ -145,24 +147,30 @@ model UploadSession {
 model LibraryEvent {
   id             BigInt      @id @default(autoincrement())
   userId         String      @db.Uuid @map("user_id")
+  cursor         BigInt
   entityType     EntityType  @map("entity_type")
   entityId       String      @db.Uuid @map("entity_id")
   action         EventAction
+  payload        Json
   payloadVersion Int         @default(1) @map("payload_version")
   createdAt      DateTime    @default(now()) @map("created_at")
   user           User        @relation(fields: [userId], references: [id], onDelete: Cascade, onUpdate: Cascade)
 
   @@index([userId])
+  @@unique([userId, cursor])
+  @@index([userId, cursor])
   @@map("library_events")
 }
 
 model DeviceSyncCursor {
-  deviceId        String   @id @db.Uuid @map("device_id")
-  lastEventId     BigInt   @default(0) @map("last_event_id")
-  lastFullSyncAt  DateTime? @map("last_full_sync_at")
-  updatedAt       DateTime @updatedAt @map("updated_at")
-  device          Device   @relation(fields: [deviceId], references: [id], onDelete: Cascade, onUpdate: Cascade)
+  deviceId   String   @id @db.Uuid @map("device_id")
+  userId     String   @db.Uuid @map("user_id")
+  cursor     BigInt   @default(0)
+  updatedAt  DateTime @updatedAt @map("updated_at")
+  device     Device   @relation(fields: [deviceId], references: [id], onDelete: Cascade, onUpdate: Cascade)
+  user       User     @relation(fields: [userId], references: [id], onDelete: Cascade, onUpdate: Cascade)
 
+  @@index([userId])
   @@map("device_sync_cursors")
 }