refactor(prisma): 将数据库从 SQLite 迁移至 PostgreSQL

删除 SQLite 相关依赖、配置和生成文件，更新 Prisma schema 以使用 PostgreSQL 作为数据源。
移除本地 dev.db 文件、libsql 适配器和旧迁移文件，简化 Prisma 客户端初始化。

backend/prisma/migrations/20260306034508_init/migration.sql

@@ -1,86 +0,0 @@
--- CreateTable
-CREATE TABLE "User" (
-    "id" TEXT NOT NULL PRIMARY KEY,
-    "name" TEXT,
-    "email" TEXT,
-    "emailVerified" DATETIME,
-    "phone" TEXT,
-    "phoneVerified" DATETIME,
-    "image" TEXT,
-    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    "updatedAt" DATETIME NOT NULL
-);
-
--- CreateTable
-CREATE TABLE "accounts" (
-    "id" TEXT NOT NULL PRIMARY KEY,
-    "user_id" TEXT NOT NULL,
-    "type" TEXT NOT NULL,
-    "provider" TEXT NOT NULL,
-    "provider_account_id" TEXT NOT NULL,
-    "refresh_token" TEXT,
-    "access_token" TEXT,
-    "expires_at" INTEGER,
-    "token_type" TEXT,
-    "scope" TEXT,
-    "id_token" TEXT,
-    "session_state" TEXT,
-    CONSTRAINT "accounts_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "User" ("id") ON DELETE CASCADE ON UPDATE CASCADE
-);
-
--- CreateTable
-CREATE TABLE "sessions" (
-    "id" TEXT NOT NULL PRIMARY KEY,
-    "session_token" TEXT NOT NULL,
-    "user_id" TEXT NOT NULL,
-    "expires" DATETIME NOT NULL,
-    CONSTRAINT "sessions_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "User" ("id") ON DELETE CASCADE ON UPDATE CASCADE
-);
-
--- CreateTable
-CREATE TABLE "posts" (
-    "id" TEXT NOT NULL PRIMARY KEY,
-    "title" TEXT NOT NULL,
-    "content" TEXT NOT NULL,
-    "user_id" TEXT NOT NULL,
-    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    "updatedAt" DATETIME NOT NULL,
-    CONSTRAINT "posts_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "User" ("id") ON DELETE CASCADE ON UPDATE CASCADE
-);
-
--- CreateTable
-CREATE TABLE "comments" (
-    "id" TEXT NOT NULL PRIMARY KEY,
-    "content" TEXT NOT NULL,
-    "user_id" TEXT NOT NULL,
-    "post_id" TEXT NOT NULL,
-    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    "updatedAt" DATETIME NOT NULL,
-    CONSTRAINT "comments_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "User" ("id") ON DELETE CASCADE ON UPDATE CASCADE,
-    CONSTRAINT "comments_post_id_fkey" FOREIGN KEY ("post_id") REFERENCES "posts" ("id") ON DELETE CASCADE ON UPDATE CASCADE
-);
-
--- CreateTable
-CREATE TABLE "verification_tokens" (
-    "identifier" TEXT NOT NULL,
-    "token" TEXT NOT NULL,
-    "expires" DATETIME NOT NULL
-);
-
--- CreateIndex
-CREATE UNIQUE INDEX "User_email_key" ON "User"("email");
-
--- CreateIndex
-CREATE UNIQUE INDEX "User_phone_key" ON "User"("phone");
-
--- CreateIndex
-CREATE UNIQUE INDEX "accounts_provider_provider_account_id_key" ON "accounts"("provider", "provider_account_id");
-
--- CreateIndex
-CREATE UNIQUE INDEX "sessions_session_token_key" ON "sessions"("session_token");
-
--- CreateIndex
-CREATE UNIQUE INDEX "verification_tokens_token_key" ON "verification_tokens"("token");
-
--- CreateIndex
-CREATE UNIQUE INDEX "verification_tokens_identifier_token_key" ON "verification_tokens"("identifier", "token");