Make city AI helpers maintainable outside scan service

The scan terminal service had grown into a 3.5k-line file that mixed endpoint orchestration, AI provider calls, fallback copy, JSON repair, and deterministic evidence guards. This extracts the city-AI helper layer into a focused module while preserving the old private names through imports for existing tests and callers.

Constraint: Keep behavior unchanged after the previous evidence-guard fixes and avoid a broad service rewrite.

Rejected: Split every scan-terminal concern at once | too much regression risk for this maintenance pass.

Confidence: high

Scope-risk: narrow

Tested: pytest tests/test_web_observability.py -q

Tested: npm run build

CHANGELOG.md

@@ -9,6 +9,7 @@
 - 城市决策卡兜底判断补充实测偏低和峰值窗口已过分支：峰后未追上模型时提示下修压力，峰前偏低时只提示等待确认
 - 城市决策卡新增过旧 METAR/观测识别：过旧报文只作为背景参考，不再触发强实况锚点、上修或下修判断；AI 缓存键同步纳入观测时间与 stale 状态
 - 城市决策卡新增 AI 结果后处理护栏：完整 DeepSeek 返回若与过旧观测、实测突破、峰后下修等确定性证据冲突，会以后端规则覆盖关键数值和结论文案
+- 后端 Scan Terminal 代码拆出 `scan_city_ai_helpers.py`，将城市 AI JSON 解析、fallback 文案、schema completion 与证据护栏从主服务文件中剥离，降低后续维护成本
 - 城市决策卡市场层改用完整 `all_buckets` 并严格识别 exact / range / or higher / or lower 温度桶方向，避免最高温中枢错配到不合理尾部桶
 - 温度桶标签统一规范化 `C/F/°C/°F`，修复 `31°°C` 这类重复单位展示
 - 决策卡展示文案将“概率差”收口为“模型-市场差”，明确口径为 `模型概率 - 市场隐含概率`