feat: implement label management with color filtering

keep-notes/prisma/migrations/20260104203746_add_labels_table/migration.sql

@@ -0,0 +1,16 @@
+-- CreateTable
+CREATE TABLE "Label" (
+    "id" TEXT NOT NULL PRIMARY KEY,
+    "name" TEXT NOT NULL,
+    "color" TEXT NOT NULL DEFAULT 'gray',
+    "userId" TEXT,
+    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" DATETIME NOT NULL,
+    CONSTRAINT "Label_userId_fkey" FOREIGN KEY ("userId") REFERENCES "User" ("id") ON DELETE CASCADE ON UPDATE CASCADE
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "Label_name_key" ON "Label"("name");
+
+-- CreateIndex
+CREATE INDEX "Label_userId_idx" ON "Label"("userId");