feat(api): 个人资料/单位/同步偏好 + 运动详情 + 身体年龄

设置 (services/settings.py, routes/settings.py)
- user_settings 表：身高/体重/出生日期/性别/单位/自动同步开关/同步频率/历史范围
- GET|PUT /api/settings，GET /api/settings/options（取值由后端给，前端不臆造）
- GET /api/settings/rating-basis：把每条参考区间的来源公开出来。
  一个把数字标成「偏低」的区间是在下判断，用户有权看到依据。

运动详情 (services/garmin.py)
- GET /api/garmin/activities/<id>/detail：概览/分段/心率区间/天气/装备/采样曲线
- 首次打开回源 Garmin 并落库，之后走缓存；?refresh=1 强制刷新
- 采样点在写入时抽稀到 300，手机图表画不了更多，也免得整行撑大

身体年龄 (services/fitness_age.py)
- 0.2.8 版 garminconnect 没有 fitnessage 接口，改为本地按公开常模推算：
  VO₂max 对应年龄为基准，静息心率与 BMI 做有上限的修正
- 返回每一步的中间值，界面照实展示，不做成一个不可追溯的分数
- 高于参考表最年轻一档时按 20 岁计——那里外推会得到「11 岁」这种结果

调度器改为每 5 分钟 tick，是否该同步按各账号自己的频率判断

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/routes/health.py

@@ -1,8 +1,12 @@
 """Health data routes: summary / steps / heart-rate / sleep / activities."""
+import datetime
+
 from flask import Blueprint, request, g, jsonify
 
 from auth import require_auth
 from services import health as health_svc
+from services import settings as settings_svc
+from services import fitness_age
 
 bp = Blueprint("health", __name__)
 
@@ -46,6 +50,33 @@ def activities():
     return jsonify(health_svc.get_activities(g.user_id, s, e))
 
 
+@bp.route("/fitness-age", methods=["GET"])
+@require_auth
+def body_age():
+    """身体年龄, computed from the profile plus the most recent readings.
+
+    VO2max only refreshes after an outdoor run or ride, so the search window
+    is wide: the last recorded value is still the current one.
+    """
+    profile = settings_svc.get_raw(g.user_id)
+    start = (datetime.date.today() - datetime.timedelta(days=180)).isoformat()
+    days = health_svc.get_summary(g.user_id, start, None)
+
+    def latest(key):
+        for day in reversed(days):
+            if day.get(key):
+                return day[key]
+        return None
+
+    return jsonify(fitness_age.estimate(
+        age=settings_svc.age_from(profile["birth_date"]),
+        sex=profile["sex"],
+        vo2max=latest("vo2max"),
+        resting_hr=latest("heartRate"),
+        bmi=settings_svc.bmi_from(profile["height_cm"], profile["weight_kg"]),
+    ))
+
+
 @bp.route("/badges", methods=["GET"])
 @require_auth
 def badges():