tests: add coverage for src/actions/ (15 files) and src/lib/{admin,auth} (3 files)

- src/actions coverage: 2.4% -> 13.55%
- src/lib/admin coverage: 44.3% -> 84.81%
- src/lib/auth coverage: 90.52%
- vitest.config.ts: exclude .next.prev/ from test discovery

src/actions/admin-media.test.ts

@@ -0,0 +1,55 @@
+import { mkdir, writeFile } from "node:fs/promises";
+import { revalidatePath } from "next/cache";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { requirePermission } from "@/lib/admin/guard";
+import { MEDIA_ROOT, resolveMediaPath } from "@/lib/media-storage";
+import { PERMS } from "@/lib/permissions";
+import { deleteMedia, uploadMedia, uploadMediaAndReturn } from "./admin-media";
+
+vi.mock("@/lib/admin/guard", () => ({ requirePermission: vi.fn() }));
+vi.mock("@/lib/permissions", () => ({ PERMS: { PAGES_EDIT: "pages.edit" } }));
+vi.mock("@/lib/media-storage", () => ({
+	MEDIA_ROOT: "/tmp/nexst-test-media",
+	resolveMediaPath: vi.fn((name: string) => `/tmp/nexst-test-media/${name}`),
+}));
+vi.mock("node:fs/promises", () => ({
+	mkdir: vi.fn(),
+	writeFile: vi.fn(),
+	unlink: vi.fn(),
+}));
+vi.mock("next/cache", () => ({ revalidatePath: vi.fn() }));
+
+const staff = { id: 1, rank: 7, username: "admin" };
+
+beforeEach(() => {
+	vi.clearAllMocks();
+	vi.mocked(requirePermission).mockResolvedValue(staff as never);
+});
+
+describe("uploadMedia", () => {
+	it("returns error when no file provided", async () => {
+		const result = await uploadMedia(new FormData());
+		expect(result.ok).toBe(false);
+		expect(result.error).toBe("No file provided");
+	});
+});
+
+describe("uploadMediaAndReturn", () => {
+	it("returns empty string when no file", async () => {
+		expect(await uploadMediaAndReturn(new FormData())).toBe("");
+	});
+});
+
+describe("deleteMedia", () => {
+	it("deletes media file and revalidates", async () => {
+		await deleteMedia("photo.png");
+		expect(revalidatePath).toHaveBeenCalledWith("/api/media");
+	});
+
+	it("skips deletion when path is outside media root", async () => {
+		vi.mocked(resolveMediaPath).mockReturnValue("/etc/passwd");
+		await deleteMedia("../../../etc/passwd");
+		const { unlink } = await import("node:fs/promises");
+		expect(unlink).not.toHaveBeenCalled();
+	});
+});