feat(scholars): add multiselect and bulk actions (delete, toggle, export)

Add Set<number>-based selection state to ScholarsPage with checkboxes
in both table and card views, select-all toggle, and stale-key pruning.

Backend: POST /scholars/bulk-delete, POST /scholars/bulk-toggle, and
GET /scholars/export?ids= filter. Service functions use user-scoped
queries. Structured logging for bulk operations.

Frontend: useScholarBulkActions composable extracts selection and bulk
action logic. AppSelect-based action dropdown with dynamic options.
Delete prompts window.confirm; enable/disable applies immediately.
Export downloads filtered JSON.

Includes integration tests for all bulk endpoints and frontend unit
tests for selection toggle, select-all, and stale pruning.

frontend/src/features/scholars/composables/useScholarBulkActions.test.ts

@@ -0,0 +1,112 @@
+// @vitest-environment happy-dom
+import { describe, expect, it, vi } from "vitest";
+import { nextTick, ref } from "vue";
+import { useScholarBulkActions, type ScholarBulkAction } from "./useScholarBulkActions";
+import type { ScholarProfile } from "@/features/scholars";
+
+vi.mock("@/features/scholars", () => ({
+  bulkDeleteScholars: vi.fn(),
+  bulkToggleScholars: vi.fn(),
+  exportScholarData: vi.fn(),
+}));
+
+function makeProfile(id: number, name: string): ScholarProfile {
+  return {
+    id,
+    scholar_id: `scholar_${id}`,
+    display_name: name,
+    profile_image_url: null,
+    profile_image_source: "none",
+    is_enabled: true,
+    baseline_completed: false,
+    last_run_dt: null,
+    last_run_status: null,
+  };
+}
+
+function setup(profiles: ScholarProfile[] = []) {
+  const visibleScholars = ref(profiles);
+  const callbacks = {
+    clearMessages: vi.fn(),
+    assignError: vi.fn(),
+    setSuccess: vi.fn(),
+    reloadScholars: vi.fn(async () => {}),
+  };
+  const bulk = useScholarBulkActions(visibleScholars, callbacks);
+  return { visibleScholars, callbacks, bulk };
+}
+
+describe("useScholarBulkActions", () => {
+  it("starts with empty selection", () => {
+    const { bulk } = setup([makeProfile(1, "Alice")]);
+    expect(bulk.selectedIds.value.size).toBe(0);
+    expect(bulk.hasSelection.value).toBe(false);
+  });
+
+  it("toggles individual row selection", () => {
+    const { bulk } = setup([makeProfile(1, "Alice"), makeProfile(2, "Bob")]);
+    bulk.onToggleRow(1, { target: { checked: true } } as unknown as Event);
+    expect(bulk.selectedIds.value.has(1)).toBe(true);
+    expect(bulk.selectedCount.value).toBe(1);
+
+    bulk.onToggleRow(1, { target: { checked: false } } as unknown as Event);
+    expect(bulk.selectedIds.value.has(1)).toBe(false);
+    expect(bulk.selectedCount.value).toBe(0);
+  });
+
+  it("toggles all visible scholars", () => {
+    const { bulk } = setup([makeProfile(1, "Alice"), makeProfile(2, "Bob")]);
+    bulk.onToggleAll({ target: { checked: true } } as unknown as Event);
+    expect(bulk.selectedIds.value.size).toBe(2);
+    expect(bulk.allVisibleSelected.value).toBe(true);
+
+    bulk.onToggleAll({ target: { checked: false } } as unknown as Event);
+    expect(bulk.selectedIds.value.size).toBe(0);
+  });
+
+  it("prunes stale selections when list changes", async () => {
+    const { visibleScholars, bulk } = setup([makeProfile(1, "Alice"), makeProfile(2, "Bob")]);
+    bulk.onToggleAll({ target: { checked: true } } as unknown as Event);
+    expect(bulk.selectedIds.value.size).toBe(2);
+
+    // Remove Bob from the visible list
+    visibleScholars.value = [makeProfile(1, "Alice")];
+    await nextTick();
+
+    expect(bulk.selectedIds.value.size).toBe(1);
+    expect(bulk.selectedIds.value.has(1)).toBe(true);
+    expect(bulk.selectedIds.value.has(2)).toBe(false);
+  });
+
+  it("shows correct bulk action options with and without selection", () => {
+    const { bulk } = setup([makeProfile(1, "Alice")]);
+    // Without selection
+    expect(bulk.bulkActionOptions.value.length).toBe(1);
+    expect(bulk.bulkActionOptions.value[0].value).toBe("select_all");
+
+    // With selection
+    bulk.onToggleRow(1, { target: { checked: true } } as unknown as Event);
+    expect(bulk.bulkActionOptions.value.length).toBe(5);
+    const values = bulk.bulkActionOptions.value.map((o) => o.value);
+    expect(values).toContain("delete_selected");
+    expect(values).toContain("enable_selected");
+    expect(values).toContain("disable_selected");
+    expect(values).toContain("export_selected");
+    expect(values).toContain("clear_selection");
+  });
+
+  it("select all action selects all visible", async () => {
+    const { bulk } = setup([makeProfile(1, "Alice"), makeProfile(2, "Bob")]);
+    bulk.bulkAction.value = "select_all" as ScholarBulkAction;
+    await bulk.onApplyBulkAction();
+    expect(bulk.selectedIds.value.size).toBe(2);
+  });
+
+  it("clear selection action clears all", async () => {
+    const { bulk } = setup([makeProfile(1, "Alice")]);
+    bulk.onToggleRow(1, { target: { checked: true } } as unknown as Event);
+    bulk.bulkAction.value = "clear_selection" as ScholarBulkAction;
+    await bulk.onApplyBulkAction();
+    expect(bulk.selectedIds.value.size).toBe(0);
+  });
+});