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/schema.prisma

@@ -2,14 +2,11 @@
 // learn more about it in the docs: https://pris.ly/d/prisma-schema
 
 generator client {
-  provider   = "prisma-client"
-  output     = "../src/generated/prisma"
-  engineType = "client"
-  runtime    = "bun"
+  provider = "prisma-client-js"
 }
 
 datasource db {
-  provider = "sqlite"
+  provider = "postgresql"
 }
 
 model User {
@@ -26,6 +23,8 @@ model User {
   sessions      Session[]
   posts         Post[]
   comments      Comment[]
+
+  @@map("users")
 }
 
 model Account {
@@ -41,7 +40,7 @@ model Account {
   scope              String?
   id_token           String?
   session_state      String?
-  user               User    @relation(fields: [userId], references: [id], onDelete: Cascade)
+  user      User    @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@unique([provider, providerAccountId])
   @@map("accounts")