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/20260309031703_add_resources_posts/migration.sql

@@ -1,30 +0,0 @@
--- CreateTable
-CREATE TABLE "resources" (
-    "id" TEXT NOT NULL PRIMARY KEY,
-    "title" TEXT NOT NULL,
-    "description" TEXT,
-    "url" TEXT NOT NULL,
-    "icon" TEXT,
-    "category" TEXT,
-    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    "updatedAt" DATETIME NOT NULL
-);
-
--- RedefineTables
-PRAGMA defer_foreign_keys=ON;
-PRAGMA foreign_keys=OFF;
-CREATE TABLE "new_posts" (
-    "id" TEXT NOT NULL PRIMARY KEY,
-    "title" TEXT NOT NULL,
-    "content" TEXT NOT NULL,
-    "user_id" TEXT NOT NULL,
-    "published" BOOLEAN NOT NULL DEFAULT false,
-    "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
-);
-INSERT INTO "new_posts" ("content", "createdAt", "id", "title", "updatedAt", "user_id") SELECT "content", "createdAt", "id", "title", "updatedAt", "user_id" FROM "posts";
-DROP TABLE "posts";
-ALTER TABLE "new_posts" RENAME TO "posts";
-PRAGMA foreign_keys=ON;
-PRAGMA defer_foreign_keys=OFF;