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/garmin.py

@@ -146,5 +146,27 @@ def sync_latest():
 @bp.route("/auto-sync", methods=["GET"])
 @require_auth
 def auto_sync_status():
-    """When the scheduler last ran and when it runs next."""
-    return jsonify(scheduler.status())
+    """When the scheduler last ran, and when this account is next due."""
+    return jsonify(scheduler.status(g.user_id))
+
+
+@bp.route("/activities/<activity_id>/detail", methods=["GET"])
+@require_auth
+def activity_detail(activity_id):
+    """Everything Garmin holds for one activity: stats, laps, zones, series.
+
+    The first open goes out to Garmin and takes a few seconds; after that it
+    is served from the stored copy. `?refresh=1` forces a refetch.
+    """
+    if not garmin_svc.has_token(g.user_id):
+        return jsonify({"error": "尚未绑定 Garmin 账号"}), 400
+
+    refresh = request.args.get("refresh") in ("1", "true", "yes")
+    try:
+        return jsonify(
+            garmin_svc.get_activity_detail(g.user_id, activity_id, refresh=refresh)
+        )
+    except garmin_svc.MFARequired as e:
+        return jsonify({"error": str(e)}), 401
+    except Exception as e:  # noqa: BLE001 - surfaced to the user verbatim
+        return jsonify({"error": garmin_svc.describe(e)}), 502