Add Prisma session store for persistent sessions

backend/prisma/migrations/20260223103350_add_session_model/migration.sql

@@ -0,0 +1,12 @@
+-- CreateTable
+CREATE TABLE "sessions" (
+    "id" TEXT NOT NULL,
+    "sid" TEXT NOT NULL,
+    "data" TEXT NOT NULL,
+    "expires_at" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "sessions_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "sessions_sid_key" ON "sessions"("sid");