feat(client): 新增会话管理与消息落库能力

补齐客户端会话生命周期接口（创建、列表、消息分页、改名、删除），并在流式 chat 中强制绑定 sessionId 与落库消息，确保会话标题和历史可追踪，同时统一 Swagger 文档为 DTO 驱动以减少重复维护。

Made-with: Cursor

prisma/migrations/20260422180000_add_chat_sessions_messages/migration.sql

@@ -0,0 +1,35 @@
+-- CreateTable
+CREATE TABLE "chat_sessions" (
+    "id" BIGSERIAL NOT NULL,
+    "user_id" BIGINT NOT NULL,
+    "title" VARCHAR(200),
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "chat_sessions_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "chat_messages" (
+    "id" BIGSERIAL NOT NULL,
+    "session_id" BIGINT NOT NULL,
+    "role" VARCHAR(20) NOT NULL,
+    "content" TEXT NOT NULL,
+    "token_count" INTEGER NOT NULL DEFAULT 0,
+    "provider" VARCHAR(64),
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "chat_messages_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "chat_sessions_user_id_idx" ON "chat_sessions"("user_id");
+
+-- CreateIndex
+CREATE INDEX "chat_messages_session_id_idx" ON "chat_messages"("session_id");
+
+-- AddForeignKey
+ALTER TABLE "chat_sessions" ADD CONSTRAINT "chat_sessions_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "users"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "chat_messages" ADD CONSTRAINT "chat_messages_session_id_fkey" FOREIGN KEY ("session_id") REFERENCES "chat_sessions"("id") ON DELETE CASCADE ON UPDATE CASCADE;