接入 OpenTelemetry 1.17.0，覆盖 HTTP、HttpClient、.NET Runtime 和数据库链路。

新增 EF Core 数据库拦截器，记录耗时、失败数、慢查询数、TraceId、查询标签和 SQL 哈希；默认不记录完整 SQL及参数。
未配置 OTLP Collector 时不启动 SDK，避免无收益的性能开销。
为课表的作息、课程、灵活课程、考试、实验等六类查询增加稳定标签。
增加可配置的 500ms 慢查询阈值，以及生产环境变量示例。
README 补充 MySQL 慢查询与 EXPLAIN ANALYZE 操作规范。

src/Jiaowu.Api/Infrastructure/Timetables/TimetableDataService.cs

@@ -28,6 +28,7 @@ public sealed class TimetableDataService(AppDbContext db)
             allowUnpublishedPlan,
             cancellationToken);
         var slots = await db.ScheduleTimeSlots.AsNoTracking()
+            .TagWith("Timetable.LoadTimeSlots")
             .Where(x => x.AcademicTermId == term.Id && x.IsEnabled)
             .OrderBy(x => x.PeriodNumber)
             .Select(x => new TimetableSlotDto(
@@ -76,6 +77,7 @@ public sealed class TimetableDataService(AppDbContext db)
             }
 
             entries = await source
+                .TagWith("Timetable.LoadScheduleEntries")
                 .OrderBy(x => x.DayOfWeek)
                 .ThenBy(x => x.StartPeriod)
                 .ThenBy(x => x.TeachingTask!.Course!.Code)
@@ -283,6 +285,7 @@ public sealed class TimetableDataService(AppDbContext db)
         }
 
         return await source
+            .TagWith("Timetable.LoadFlexibleCourses")
             .OrderBy(x => x.Course!.Code)
             .ThenBy(x => x.TaskNumber)
             .Select(x => new FlexibleCourseDto(
@@ -326,6 +329,7 @@ public sealed class TimetableDataService(AppDbContext db)
             legacyQuery, resourceType, resourceId, studentId);
 
         var legacySessions = await legacyQuery
+            .TagWith("Timetable.LoadLegacyExamEntries")
             .OrderBy(x => x.ExamDate)
             .ThenBy(x => x.StartPeriod)
             .Select(x => new ExamSessionProjection(
@@ -416,6 +420,7 @@ public sealed class TimetableDataService(AppDbContext db)
             db,
             studentId.Value);
         var sessions = await db.ExperimentSessions.AsNoTracking()
+            .TagWith("Timetable.LoadExperimentEntries")
             .AsSplitQuery()
             .Where(x =>
                 x.ExperimentProject!.TeachingTask!.AcademicTermId == term.Id &&