chore: 数据已迁到 SQLite，删掉 Postgres 迁移脚本、webpreview-db 服务和迁移文档

school 服务器以后要切换时，从 cdeb7c1 里找回（deploy.yaml 里写了）。
备份方法挪到 CLAUDE.md。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/scripts/migrate-from-postgres.ts

@@ -1,134 +0,0 @@
-/**
- * 把旧 Django 项目的 Postgres 数据导入 SQLite（一次性迁移）。
- *
- *   POSTGRES_HOST=... POSTGRES_DB=... POSTGRES_USER=... POSTGRES_PASSWORD=... \
- *     bun scripts/migrate-from-postgres.ts [--force]
- *
- * 目标库为 DB_PATH（默认 data/db.sqlite3）。目标库已有数据时需要 --force（会先清空）。
- * 会话不迁移，所有人需要重新登录一次。
- */
-import { SQL } from "bun"
-import { config } from "../src/config"
-import { db } from "../src/db"
-
-// 按外键依赖顺序
-const TABLES = [
-  "account_user",
-  "task_task",
-  "task_tutorial",
-  "task_challenge",
-  "task_creation",
-  "task_taskasset",
-  "submission_submission",
-  "submission_rating",
-  "submission_award",
-  "submission_submissionaward",
-  "prompt_conversation",
-  "prompt_message",
-]
-
-const force = process.argv.includes("--force")
-const env = process.env
-for (const key of ["POSTGRES_HOST", "POSTGRES_DB", "POSTGRES_USER", "POSTGRES_PASSWORD"]) {
-  if (!env[key]) {
-    console.error(`缺少环境变量 ${key}`)
-    process.exit(1)
-  }
-}
-
-const pg = new SQL({
-  hostname: env.POSTGRES_HOST,
-  port: Number(env.POSTGRES_PORT ?? 5432),
-  database: env.POSTGRES_DB,
-  username: env.POSTGRES_USER,
-  password: env.POSTGRES_PASSWORD,
-})
-
-const existing = db.query(`SELECT COUNT(*) AS n FROM account_user`).get() as { n: number }
-if (existing.n > 0 && !force) {
-  console.error(`目标库 ${config.dbPath} 已有数据，加 --force 会清空后重新导入`)
-  process.exit(1)
-}
-
-const quote = (name: string) => `"${name.replaceAll('"', '""')}"`
-
-// 按 Postgres 的列类型转换成 Django 在 SQLite 中的存储格式
-function selectExpr(column: string, pgType: string) {
-  const c = quote(column)
-  switch (pgType) {
-    case "timestamp with time zone":
-      return `to_char(${c} AT TIME ZONE 'UTC', 'YYYY-MM-DD HH24:MI:SS.US') AS ${c}`
-    case "timestamp without time zone":
-      return `to_char(${c}, 'YYYY-MM-DD HH24:MI:SS.US') AS ${c}`
-    case "uuid":
-      return `replace(${c}::text, '-', '') AS ${c}`
-    case "boolean":
-      return `${c}::int AS ${c}`
-    case "bigint":
-      return `${c}::text AS ${c}`
-    default:
-      return c
-  }
-}
-
-const counts: Record<string, number> = {}
-const rowsByTable: Record<string, Record<string, unknown>[]> = {}
-
-for (const table of TABLES) {
-  const sqliteColumns = (db.query(`PRAGMA table_info(${quote(table)})`).all() as { name: string }[]).map(
-    (c) => c.name,
-  )
-  const pgColumns = await pg`
-    SELECT column_name, data_type FROM information_schema.columns
-    WHERE table_schema = 'public' AND table_name = ${table}`
-  if (!pgColumns.length) throw new Error(`Postgres 中没有表 ${table}`)
-  const pgTypes = new Map<string, string>(pgColumns.map((c: any) => [c.column_name, c.data_type]))
-  const missing = sqliteColumns.filter((c) => !pgTypes.has(c))
-  if (missing.length) throw new Error(`Postgres 表 ${table} 缺少列：${missing.join(", ")}`)
-
-  const select = sqliteColumns.map((c) => selectExpr(c, pgTypes.get(c)!)).join(", ")
-  const rows: Record<string, unknown>[] = await pg.unsafe(`SELECT ${select} FROM ${quote(table)}`)
-  for (const row of rows) {
-    for (const c of sqliteColumns) {
-      if (pgTypes.get(c) === "bigint" && row[c] !== null) row[c] = Number(row[c])
-    }
-  }
-  rowsByTable[table] = rows
-  console.log(`读取 ${table}: ${rows.length} 行`)
-}
-await pg.close()
-
-db.transaction(() => {
-  db.run("PRAGMA defer_foreign_keys = ON")
-  for (const table of [...TABLES].reverse()) db.run(`DELETE FROM ${quote(table)}`)
-  db.run(`DELETE FROM auth_refresh_token`)
-  for (const table of TABLES) {
-    const rows = rowsByTable[table]!
-    if (!rows.length) {
-      counts[table] = 0
-      continue
-    }
-    const columns = Object.keys(rows[0]!)
-    const insert = db.prepare(
-      `INSERT INTO ${quote(table)} (${columns.map(quote).join(", ")})
-       VALUES (${columns.map((c) => "$" + c).join(", ")})`,
-    )
-    for (const row of rows) insert.run(row as any)
-    counts[table] = rows.length
-  }
-})()
-
-// 校验
-let ok = true
-for (const table of TABLES) {
-  const n = (db.query(`SELECT COUNT(*) AS n FROM ${quote(table)}`).get() as { n: number }).n
-  if (n !== counts[table]) ok = false
-  console.log(`${n === counts[table] ? "✓" : "✗"} ${table}: ${n}`)
-}
-const fk = db.query(`PRAGMA foreign_key_check`).all()
-if (fk.length) {
-  ok = false
-  console.error("外键检查失败：", fk.slice(0, 10))
-}
-console.log(ok ? `\n导入完成：${config.dbPath}` : "\n导入有问题，请检查上面的输出")
-process.exit(ok ? 0 : 1)