feat: implement multi-identifier architecture (Issue #496)

- Database: Add user_login_ids table for 1:N identifier mapping and remove legacy login_id column
- Kratos: Update identity schema to use custom_login_ids array instead of a single id trait
- Backend: Implement syncCustomLoginIDs to collect isLoginId fields across tenant schemas
- Backend: Add backtracking logic to auto-assign session tenant based on used login identifier
- Backend: Add 409 Conflict exception handling for Create/Update operations
- AdminFront: Refactor UserDetailPage to a tabbed grid layout (Info, Tenants, Security)
- AdminFront: Show '로그인 ID' badge on tenant schema fields used for authentication
- UserFront: Remove legacy optional 'Login ID' input from signup flow
- Tests: Add multi-identifier repository tests and update handler tests

backend/internal/repository/user_repository_test.go

@@ -94,4 +94,54 @@ func TestUserRepository(t *testing.T) {
 		assert.Equal(t, int64(1), counts["tenant-b"])
 		assert.Equal(t, int64(0), counts["tenant-c"])
 	})
+
+	t.Run("Multi-Identifier Support", func(t *testing.T) {
+		_ = testDB.AutoMigrate(&domain.UserLoginID{})
+		testDB.Exec("DELETE FROM user_login_ids")
+		testDB.Exec("DELETE FROM users")
+
+		user := &domain.User{Email: "multi@test.com", Name: "Multi"}
+		_ = repo.Create(ctx, user)
+
+		t1 := "00000000-0000-0000-0000-000000000001"
+		t2 := "00000000-0000-0000-0000-000000000002"
+
+		loginIDs := []domain.UserLoginID{
+			{UserID: user.ID, TenantID: t1, FieldKey: "emp_id", LoginID: "E001"},
+			{UserID: user.ID, TenantID: t2, FieldKey: "student_id", LoginID: "S001"},
+		}
+
+		err := repo.UpdateUserLoginIDs(ctx, user.ID, loginIDs)
+		assert.NoError(t, err)
+
+		// Get and Verify
+		saved, err := repo.GetUserLoginIDs(ctx, user.ID)
+		assert.NoError(t, err)
+		assert.Len(t, saved, 2)
+
+		// IsLoginIDTaken
+		taken, err := repo.IsLoginIDTaken(ctx, "E001")
+		assert.NoError(t, err)
+		assert.True(t, taken)
+
+		taken, err = repo.IsLoginIDTaken(ctx, "UNKNOWN")
+		assert.NoError(t, err)
+		assert.False(t, taken)
+
+		// FindTenantIDByLoginID
+		tid, err := repo.FindTenantIDByLoginID(ctx, "S001")
+		assert.NoError(t, err)
+		assert.Equal(t, t2, tid)
+
+		// Update (Replace)
+		newList := []domain.UserLoginID{
+			{UserID: user.ID, TenantID: t1, FieldKey: "emp_id", LoginID: "E002"},
+		}
+		err = repo.UpdateUserLoginIDs(ctx, user.ID, newList)
+		assert.NoError(t, err)
+
+		saved, _ = repo.GetUserLoginIDs(ctx, user.ID)
+		assert.Len(t, saved, 1)
+		assert.Equal(t, "E002", saved[0].LoginID)
+	})
 }