allow importing of libraries via URL, update db schema

backend/prisma/migrations/20251124220546_add_library_model/migration.sql

@@ -0,0 +1,7 @@
+-- CreateTable
+CREATE TABLE "Library" (
+    "id" TEXT NOT NULL PRIMARY KEY DEFAULT 'default',
+    "items" TEXT NOT NULL DEFAULT '[]',
+    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" DATETIME NOT NULL
+);

backend/prisma/schema.prisma

@@ -33,3 +33,10 @@ model Drawing {
   createdAt    DateTime    @default(now())
   updatedAt    DateTime    @updatedAt
 }
+
+model Library {
+  id        String   @id @default("default") // Singleton pattern - use "default" ID
+  items     String   @default("[]") // Stored as JSON string array of library items
+  createdAt DateTime @default(now())
+  updatedAt DateTime @updatedAt
+}