ci: 运行环境自愈预置 + 发版任务串行 + 删 ci.yml/暂停备份

- 新增 scripts/ci/provision-{mac,windows}.sh：幂等预置，按 Flutter 版本
  stamp 短路，pipeline 首跑自动 precache + 预热缓存，运行时不再下载 engine
- scripts/ci/_env.sh：抽出共用镜像 env，compile 脚本统一 source
- deploy.yml：build-macos needs build-linux-web，同一 mac runner 串行
  执行，避免排队超时；各 build job 加 Provision 前置步骤
- 删除 .gitea/workflows/ci.yml
- backup.yml：暂停定时备份（保留 workflow_dispatch 手动触发）

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/ci/_env.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# _env.sh — shared CI mirror env. `source` this from other scripts.
+# Idempotent: only sets vars if not already provided by the environment.
+
+export GOPROXY="${GOPROXY:-https://goproxy.cn,direct}"
+export PUB_HOSTED_URL="${PUB_HOSTED_URL:-https://pub.flutter-io.cn}"
+export FLUTTER_STORAGE_BASE_URL="${FLUTTER_STORAGE_BASE_URL:-https://storage.flutter-io.cn}"
+
+# Ensure Homebrew tools are on PATH (macOS runners); harmless elsewhere.
+case ":${PATH}:" in
+  *":/opt/homebrew/bin:"*) ;;
+  *) export PATH="/opt/homebrew/bin:${PATH}" ;;
+esac