feat: update database schema for multi-user support

- Add User model with email, passwordHash, and name fields
- Add Collaborator model for list sharing with permission levels
- Add Invitation model for sharing invitations with status tracking
- Add ownerId field to List model with cascade deletion
- Add necessary indexes for performance optimization
- Implements Requirements 1.1, 4.1, 5.1

prisma/schema.prisma

@@ -7,13 +7,71 @@ datasource db {
   url      = env("DATABASE_URL")
 }
 
+model User {
+  id                   String       @id @default(cuid())
+  email                String       @unique
+  name                 String?
+  passwordHash         String
+  createdAt            DateTime     @default(now())
+  updatedAt            DateTime     @updatedAt
+  ownedLists           List[]       @relation("ListOwner")
+  collaborations       Collaborator[]
+  sentInvitations      Invitation[] @relation("InvitationInviter")
+  receivedInvitations  Invitation[] @relation("InvitationReceiver")
+
+  @@index([email])
+}
+
 model List {
-  id        String   @id @default(cuid())
-  title     String
-  color     String   @default("#3B82F6")
-  createdAt DateTime @default(now())
-  updatedAt DateTime @updatedAt
-  items     Item[]
+  id            String         @id @default(cuid())
+  title         String
+  color         String         @default("#3B82F6")
+  ownerId       String
+  createdAt     DateTime       @default(now())
+  updatedAt     DateTime       @updatedAt
+  owner         User           @relation("ListOwner", fields: [ownerId], references: [id], onDelete: Cascade)
+  items         Item[]
+  collaborators Collaborator[]
+  invitations   Invitation[]
+
+  @@index([ownerId])
+  @@index([ownerId, createdAt])
+}
+
+model Collaborator {
+  id         String   @id @default(cuid())
+  userId     String
+  listId     String
+  permission String   // 'editor' or 'viewer'
+  createdAt  DateTime @default(now())
+  user       User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  list       List     @relation(fields: [listId], references: [id], onDelete: Cascade)
+
+  @@unique([userId, listId])
+  @@index([userId])
+  @@index([listId])
+  @@index([listId, permission])
+}
+
+model Invitation {
+  id         String   @id @default(cuid())
+  email      String
+  listId     String
+  inviterId  String
+  permission String   // 'editor' or 'viewer'
+  status     String   @default("pending") // 'pending', 'accepted', 'declined'
+  createdAt  DateTime @default(now())
+  expiresAt  DateTime
+  list       List     @relation(fields: [listId], references: [id], onDelete: Cascade)
+  inviter    User     @relation("InvitationInviter", fields: [inviterId], references: [id], onDelete: Cascade)
+  receiver   User?    @relation("InvitationReceiver", fields: [email], references: [email], onDelete: Cascade)
+
+  @@index([email])
+  @@index([listId])
+  @@index([inviterId])
+  @@index([status])
+  @@index([email, status])
+  @@index([listId, status])
 }
 
 model Item {