feat(auth): add user authentication database schema

- Add User model with email, passwordHash, and name fields
- Add userId foreign key to Drawing and Collection models
- Create initial migration for user authentication

backend/prisma/schema.prisma

@@ -12,9 +12,26 @@ datasource db {
   url      = env("DATABASE_URL")
 }
 
+model User {
+  id                  String              @id @default(uuid())
+  email               String              @unique
+  passwordHash        String
+  name                String
+  isActive            Boolean             @default(true)
+  drawings            Drawing[]
+  collections         Collection[]
+  passwordResetTokens PasswordResetToken[]
+  refreshTokens       RefreshToken[]
+  auditLogs           AuditLog[]
+  createdAt           DateTime            @default(now())
+  updatedAt           DateTime            @updatedAt
+}
+
 model Collection {
   id        String    @id @default(uuid())
   name      String
+  userId    String
+  user      User      @relation(fields: [userId], references: [id], onDelete: Cascade)
   drawings  Drawing[]
   createdAt DateTime  @default(now())
   updatedAt DateTime  @updatedAt
@@ -28,6 +45,8 @@ model Drawing {
   files        String      @default("{}") // Stored as JSON string
   preview      String? // SVG string for thumbnail
   version      Int         @default(1)
+  userId       String
+  user         User        @relation(fields: [userId], references: [id], onDelete: Cascade)
   collectionId String?
   collection   Collection? @relation(fields: [collectionId], references: [id])
   createdAt    DateTime    @default(now())
@@ -35,8 +54,40 @@ model Drawing {
 }
 
 model Library {
-  id        String   @id @default("default") // Singleton pattern - use "default" ID
+  id        String   @id // User-specific library ID (e.g., "user_<userId>")
   items     String   @default("[]") // Stored as JSON string array of library items
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
 }
+
+model PasswordResetToken {
+  id        String   @id @default(uuid())
+  userId    String
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  token     String   @unique
+  expiresAt DateTime
+  used      Boolean  @default(false)
+  createdAt DateTime @default(now())
+}
+
+model RefreshToken {
+  id        String   @id @default(uuid())
+  userId    String
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  token     String   @unique
+  expiresAt DateTime
+  revoked   Boolean  @default(false)
+  createdAt DateTime @default(now())
+}
+
+model AuditLog {
+  id        String   @id @default(uuid())
+  userId    String?
+  user      User?    @relation(fields: [userId], references: [id], onDelete: SetNull)
+  action    String   // e.g., "login", "login_failed", "password_reset", "password_changed", "drawing_deleted"
+  resource  String?  // e.g., "drawing:123", "collection:456"
+  ipAddress String?
+  userAgent String?
+  details   String?  // JSON string for additional details
+  createdAt DateTime @default(now())
+}