fix: 修复后端 JSON 反序列化与循环引用问题

- Program.cs 添加 JsonStringEnumConverter + CamelCase 策略，
  修复枚举值和 decimal 反序列化失败
- CreateHealthRecordRequest 改为 class + init 属性，
  修复 decimal? 在位置记录中的 JSON 反序列化异常
- 运动计划 /current 端点改用匿名对象投影，
  修复 EF Core 导航属性循环引用导致 JSON 无限递归

backend/src/Health.WebApi/Endpoints/remaining_endpoints.cs

@@ -193,7 +193,21 @@ public static class RemainingEndpoints
             var today = DateOnly.FromDateTime(DateTime.Now);
             var monday = today.AddDays(-(int)today.DayOfWeek + 1);
             var plan = await db.ExercisePlans.Include(p => p.Items).FirstOrDefaultAsync(p => p.UserId == userId && p.WeekStartDate == monday);
-            return Results.Ok(new { code = 0, data = plan, message = (string?)null });
+            if (plan == null) return Results.Ok(new { code = 0, data = (object?)null, message = (string?)null });
+            return Results.Ok(new
+            {
+                code = 0,
+                data = new
+                {
+                    plan.Id, plan.WeekStartDate, plan.CreatedAt, plan.UpdatedAt,
+                    items = plan.Items.Select(i => new
+                    {
+                        i.Id, i.DayOfWeek, i.ExerciseType, i.DurationMinutes,
+                        i.IsCompleted, i.CompletedAt, i.IsRestDay
+                    })
+                },
+                message = (string?)null
+            });
         });
 
         group.MapPost("/", async (CreateExercisePlanRequest req, HttpContext http, AppDbContext db, CancellationToken ct) =>