feat(gradebook): import external assessment grades from CSV

Introduce the full external-assessment CSV import feature as a single
slice on top of grade validation (pr3).

Backend:
- Course::Gradebook::ExternalAssessmentImportService — header-order
  tolerance, duplicate-header/identifier guards, reassigned-identifier
  detection, out-of-range flagging, and batched (bulk insert + upsert)
  grade writes.
- external_assessment_imports controller (preview + create/commit),
  create/preview jbuilders, and import routes.

Frontend:
- ImportExternalAssessmentsWizard (upload → define → verify), with the
  ExternalGradeConflict prompt/table change-matrix and buildTemplate.
- previewImport/commitImport operations + import API endpoints + types.
- ManageExternalAssessmentsPanel gains the Import CSV entry point.

Consolidates the previously planned pr4a (correctness) and pr4b (ux)
into one PR: the import wizard and service are edited by both themes at
the hunk level, so they do not separate into independently-reviewable
slices.

Fixes a pre-existing broken assertion in the import controller spec
(course_externalassessments -> course_external_assessments) that
prevented the replace-on-conflict test from running.

Author: LWS49

app/controllers/course/external_assessment_imports_controller.rb

@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+class Course::ExternalAssessmentImportsController < Course::ComponentController
+  Service = Course::Gradebook::ExternalAssessmentImportService
+
+  def preview
+    authorize! :manage_gradebook_weights, current_course
+    @result = build_service.preview
+    render 'preview'
+  rescue Service::ImportError => e
+    render json: { errors: e.payload }, status: :unprocessable_entity
+  end
+
+  def create
+    authorize! :manage_gradebook_weights, current_course
+    @summary = build_service.commit(on_conflict: import_params[:onConflict])
+    render 'create'
+  rescue Service::ImportError => e
+    render json: { errors: e.payload }, status: :unprocessable_entity
+  end
+
+  private
+
+  def component
+    current_component_host[:course_gradebook_component]
+  end
+
+  def build_service
+    permitted_params = import_params
+    weighted_view_enabled = gradebook_settings.weighted_view_enabled
+    Service.new(
+      course: current_course,
+      actor: current_user,
+      components: permitted_params[:components].map do |c|
+        { name: c[:name],
+          weightage: weighted_view_enabled ? c[:weightage].to_i : 0,
+          maximum_grade: c[:maximumGrade].to_f }
+      end,
+      identifier_mode: permitted_params[:identifierMode],
+      csv_data: permitted_params[:csvData]
+    )
+  end
+
+  def gradebook_settings
+    @gradebook_settings ||= Course::Settings::GradebookComponent.new(component)
+  end
+
+  def import_params
+    @import_params ||= params.slice(:identifierMode, :csvData, :onConflict, :components).permit(
+      :identifierMode, :csvData, :onConflict,
+      components: [:name, :weightage, :maximumGrade]
+    )
+  end
+end