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/settings.py

@@ -0,0 +1,85 @@
+"""Profile, units and sync preferences."""
+from flask import Blueprint, request, g, jsonify
+
+from auth import require_auth
+from services import settings as settings_svc
+from services import fitness_age
+
+bp = Blueprint("settings", __name__)
+
+
+@bp.route("", methods=["GET"])
+@bp.route("/", methods=["GET"])
+@require_auth
+def get_settings():
+    return jsonify(settings_svc.get_settings(g.user_id))
+
+
+@bp.route("", methods=["PUT"])
+@bp.route("/", methods=["PUT"])
+@require_auth
+def put_settings():
+    try:
+        return jsonify(settings_svc.save_settings(g.user_id, request.json or {}))
+    except settings_svc.InvalidSetting as e:
+        return jsonify({"error": str(e)}), 400
+
+
+@bp.route("/options", methods=["GET"])
+@require_auth
+def options():
+    """The values the pickers may offer, so the UI never invents one the
+    backend would reject."""
+    return jsonify({
+        "sexes": list(settings_svc.SEXES),
+        "units": list(settings_svc.UNITS),
+        "autoSyncMinutes": list(settings_svc.INTERVALS),
+        "historyDays": list(settings_svc.HISTORY),
+    })
+
+
+@bp.route("/rating-basis", methods=["GET"])
+@require_auth
+def rating_basis():
+    """Where every rating in the app comes from.
+
+    Surfaced in 设置 because a band that colours a number 偏低 is making a
+    claim, and the user is entitled to see what that claim rests on.
+    """
+    return jsonify({
+        "fitnessAge": fitness_age.BASIS,
+        "bands": BAND_SOURCES,
+        "note": (
+            "参考区间用于给数字一个位置感，不是诊断标准。"
+            "AI 只负责解读这些结果，不参与设定任何阈值。"
+        ),
+    })
+
+
+# Kept beside the UI's RANGES table (client/src/lib/ranges.ts). Each entry says
+# where that metric's band edges came from — the honest answer for several of
+# them is "general-population orientation figures", and it says so.
+BAND_SOURCES = [
+    {"metric": "步数", "bands": "<5k 偏低 · 5–8k 一般 · 8–12k 达标 · >12k 优秀",
+     "source": "步数与死亡率的队列研究（约 8000 步起获益明显，12000 步后趋平）"},
+    {"metric": "静息心率", "bands": "<50 很低 · 50–65 正常 · 65–75 偏高 · >75 较高",
+     "source": "健康成人静息心率 60–100 bpm 为正常范围，规律运动者常低于 60"},
+    {"metric": "心率变异性", "bands": "<25 偏低 · 25–40 一般 · 40–70 良好 · >70 很好",
+     "source": "夜间 RMSSD 的一般人群分布；个体差异极大，趋势比绝对值更有意义"},
+    {"metric": "睡眠时长", "bands": "<6h 不足 · 6–7h 偏少 · 7–9h 充足 · >9h 偏多",
+     "source": "美国睡眠医学会 / 睡眠研究会成人 7–9 小时建议"},
+    {"metric": "压力", "bands": "0–25 休息 · 26–50 偏低 · 51–75 中等 · >75 偏高",
+     "source": "Garmin 官方压力分级，与手表显示一致"},
+    {"metric": "身体电量", "bands": "0–25 很低 · 26–50 偏低 · 51–75 良好 · >75 充足",
+     "source": "Garmin 官方 Body Battery 分级"},
+    {"metric": "血氧", "bands": "<90 偏低 · 90–94 略低 · ≥95 正常",
+     "source": "静息血氧饱和度常用临床参考；腕表光学测量误差较大，仅供趋势参考"},
+    {"metric": "呼吸频率", "bands": "<12 偏低 · 12–20 正常 · >20 偏高",
+     "source": "成人静息呼吸频率 12–20 次/分"},
+    {"metric": "强度分钟", "bands": "<10 偏少 · 10–21 一般 · ≥21 达标",
+     "source": "WHO 每周 150 分钟中等强度活动，折合每天约 21 分钟"},
+    {"metric": "训练准备度", "bands": "0–25 很低 · 26–50 偏低 · 51–75 就绪 · >75 很好",
+     "source": "Garmin 官方 Training Readiness 分级"},
+    {"metric": "爬楼", "bands": "<5 偏少 · 5–10 达标 · >10 优秀",
+     "source": "一般性活动量参考，无权威标准"},
+]