feat: implement label management with color filtering

keep-notes/prisma/schema.prisma

@@ -19,6 +19,7 @@ model User {
   accounts      Account[]
   sessions      Session[]
   notes         Note[]
+  labels        Label[]
   createdAt     DateTime  @default(now())
   updatedAt     DateTime  @updatedAt
 }
@@ -62,6 +63,18 @@ model VerificationToken {
   @@id([identifier, token])
 }
 
+model Label {
+  id        String   @id @default(cuid())
+  name      String   @unique
+  color     String   @default("gray")
+  userId    String?
+  user      User?    @relation(fields: [userId], references: [id], onDelete: Cascade)
+  createdAt DateTime @default(now())
+  updatedAt DateTime @updatedAt
+
+  @@index([userId])
+}
+
 model Note {
   id                  String    @id @default(cuid())
   title               String?