docs: 交付文档、维护指引与端到端回归

补齐 README、开发计划、维护交接与 E2E 用例，执行最终验收。

e2e/src/support/db.ts

@@ -0,0 +1,18 @@
+import { execFileSync } from 'node:child_process'
+import { env } from '../env'
+
+/**
+ * Direct SQL on the throwaway e2e database, for state no API sets — e.g. moving
+ * a timestamp past a deadline a worker checks. Keep it rare; seed through src/api.
+ */
+export function sql(query: string): string {
+  return execFileSync('psql', [env.databaseUrl, '-v', 'ON_ERROR_STOP=1', '-qtA', '-c', query], { encoding: 'utf8' }).trim()
+}
+
+const UUID = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i
+
+/** Guards every id interpolated into SQL. */
+export function uuid(value: string): string {
+  if (!UUID.test(value)) throw new Error(`not a UUID: ${value}`)
+  return value
+}