feat: add course roster repository methods

AGENTS.md

@@ -69,7 +69,7 @@ MVVM pattern with Riverpod for DI and reactive state (manual providers, no codeg
 
 - AuthRepository — User identity, session, profile. Lazy auth via `withAuth<T>()` with SSO and re-auth coalescing (Completer pattern). Session persistence via flutter_secure_storage. Never-completing future on auth failure (harmless — session-scoped providers are already being disposed).
 - PreferencesRepository — Typed `PrefKey<T>` enum with SharedPreferencesAsync. Cloud sync via avatar payload.
-- CourseRepository — Course catalog, schedules, and offering details. Normalizes bilingual names from multiple sources (catalog vs offering). Layout computation for course table grid (multi-period spans, noon-crossing, unscheduled courses).
+- CourseRepository — Course catalog, schedules, offering details, and related I-School Plus data. Normalizes bilingual names from multiple sources (catalog vs offering). Layout computation for course table grid (multi-period spans, noon-crossing, unscheduled courses).
 - CalendarRepository — Academic calendar events from NTUT portal. Sliding window caching keyed to enrolled semesters.
 - StudentRepository — Academic records, GPA, rankings. Parallel course code resolution via CourseRepository.getCourse().
 - **Method pattern:** `watchX()` returns a `Stream` backed by Drift `.watch()` — emits cached data immediately, then background-fetches if empty or stale (each method has its own hard-coded TTL `const`). Network errors are absorbed (stale data preferred over errors). `refreshX()` is the imperative counterpart for pull-to-refresh — fetches from network, writes to DB, and lets the stream re-emit.
@@ -146,4 +146,4 @@ These apOu codes are the SSO target identifiers used by PortalService to obtain
 
 ## Backlog
 
-Open work is tracked in [GitHub Issues](https://github.com/NTUT-NPC/tattoo/issues). Key areas: remaining NTUT service methods (ISchoolPlus announcements, StudentQuery extensions), repository layer gaps (materials, rosters), and file download infrastructure.
+Open work is tracked in [GitHub Issues](https://github.com/NTUT-NPC/tattoo/issues). Key areas: remaining NTUT service methods (ISchoolPlus announcements, StudentQuery extensions), repository layer gaps (materials), and file download infrastructure.

lib/database/actions.dart

@@ -251,4 +251,24 @@ extension DatabaseActions on AppDatabase {
       ),
     )).id;
   }
+
+  /// Returns the ID of an existing student row, or creates/updates one.
+  ///
+  /// A null [name] never overwrites a previously recorded name — some
+  /// I-School Plus rosters omit the name for certain students.
+  Future<int> upsertStudent({
+    required String studentId,
+    String? name,
+  }) async {
+    return (await into(students).insertReturning(
+      StudentsCompanion.insert(
+        studentId: studentId,
+        name: Value(name),
+      ),
+      onConflict: DoUpdate(
+        (old) => StudentsCompanion(name: .absentIfNull(name)),
+        target: [students.studentId],
+      ),
+    )).id;
+  }
 }

lib/database/schema.dart

@@ -425,6 +425,10 @@ class CourseOfferings extends Table with AutoIncrementId, Fetchable {
   /// Teacher-authored remarks from the syllabus page (備註).
   late final syllabusRemarks = text().nullable()();
 
+  /// When the I-School Plus roster (classmates) was last fetched for this
+  /// offering. Null until the first roster fetch.
+  late final rosterFetchedAt = dateTime().nullable()();
+
   @override
   List<Set<Column>> get uniqueKeys => [
     {semester, number},