test: reorganize flat test files into purpose-based subdirectories (phase 6)

Group 245 flat integration tests and 25 loose unit tests into
discoverable subdirectories per spec section 6:

  integration/tools/{image,video,audio,document,data}/  (156 files)
  integration/platform/                                  (64 files)
  integration/generated/                                 (14 files)
  integration/security/                                  (10 files)

  unit/security/     (8 files, new subdir)
  unit/api/          (7 files moved in)
  unit/web/          (3 files moved in)
  unit/shared/       (6 files moved in)
  unit/image-engine/ (1 file moved in)

All moves via git mv (history preserved). Relative imports repaired
for both depth levels (platform/generated/security = +1, tools/ = +2):
static from-imports, dynamic import() calls, vi.mock() paths,
import.meta.dirname joins, and __dirname joins.

Vitest discovery unchanged (no test.include in config, recursive glob
matches subdirs, shard-by-hash unaffected). test-server.ts and
tool-route-drift.test.ts stay at integration root. fixtures/ untouched.

Parity gate: 13189 passing test names before = 13189 after (0 dropped).

tests/integration/tools/document/linearize-pdf.test.ts

@@ -0,0 +1,64 @@
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { qpdfAvailable, qpdfPageCount } from "@snapotter/doc-engine";
+import { afterAll, beforeAll, describe, expect, it } from "vitest";
+import { fixtures, readFixture } from "../../../fixtures/index.js";
+import {
+  buildTestApp,
+  createMultipartPayload,
+  loginAsAdmin,
+  type TestApp,
+} from "../../test-server.js";
+
+const PDF = readFixture(fixtures.document.pdf3);
+
+let testApp: TestApp;
+let adminToken: string;
+
+beforeAll(async () => {
+  testApp = await buildTestApp();
+  adminToken = await loginAsAdmin(testApp.app);
+}, 30_000);
+
+afterAll(async () => {
+  await testApp.cleanup();
+}, 10_000);
+
+async function runTool() {
+  const { body, contentType } = createMultipartPayload([
+    { name: "file", filename: "test-3page.pdf", contentType: "application/pdf", content: PDF },
+    { name: "settings", content: JSON.stringify({}) },
+  ]);
+  return testApp.app.inject({
+    method: "POST",
+    url: "/api/v1/tools/linearize-pdf",
+    headers: { authorization: `Bearer ${adminToken}`, "content-type": contentType },
+    body,
+  });
+}
+
+describe.skipIf(!qpdfAvailable())("linearize-pdf (requires qpdf)", () => {
+  it("linearizes a PDF preserving page count", async () => {
+    const res = await runTool();
+    expect(res.statusCode).toBe(200);
+    const envelope = JSON.parse(res.body);
+    expect(envelope.downloadUrl).toBeDefined();
+
+    const dl = await testApp.app.inject({
+      method: "GET",
+      url: envelope.downloadUrl,
+    });
+    expect(dl.statusCode).toBe(200);
+
+    const dir = mkdtempSync(join(tmpdir(), "linearize-pdf-test-"));
+    try {
+      const outPath = join(dir, "linearized.pdf");
+      writeFileSync(outPath, dl.rawPayload);
+      const pages = await qpdfPageCount(outPath);
+      expect(pages).toBe(3);
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  }, 60_000);
+});