fix(import): surface real error cause in /pages/import instead of generic 400

The two catch blocks in importPage() threw an opaque "Error processing file
content" / "Failed to create imported page" BadRequest, hiding the real cause
from the HTTP response. This made a production 400 regression impossible to
diagnose without server log access, and violated the project convention that
errors must never be swallowed.

Extract `${err.name}: ${err.message}` into both the log (full err object kept
for the stack) and the thrown BadRequestException. Inner processMarkdown/
processHTML rethrowing catches and the EE processDocx/processPdf license
catches are left unchanged.

Local reproduction of the happy-dom 14->20 theory failed (full import chain
+ 22 edge cases pass on happy-dom@20.8.9), so the root cause is still pending
the now-visible reason from a recurring 400. Diagnostic script test-import.tsx
added; backlog doc updated with findings.

test-import.tsx

@@ -0,0 +1,111 @@
+// Diagnostic / reproduction script for the /pages/import 400 regression
+// (see docs/backlog/pages-import-broken-400.md).
+//
+// Run from repo root:  npx tsx test-import.tsx
+//
+// Exercises the full server-side import chain directly against source:
+//   markdownToHtml (@docmost/editor-ext)
+//   -> cheerio load + normalizeImportHtml
+//   -> generateJSON (happy-dom DOMParser -> ProseMirror) with all 44 tiptapExtensions
+//
+// Also pokes the happy-dom cleanup behavior used in generateJSON's `finally`
+// block, to rule out the "finally throw masks the real result" footgun.
+//
+// If this script throws on some input, that input reproduces the prod 400 and
+// the thrown error is the real cause hidden behind "Error processing file content".
+
+import { markdownToHtml } from '@docmost/editor-ext';
+import { generateJSON } from './apps/server/src/common/helpers/prosemirror/html/generateJSON';
+import { tiptapExtensions } from './apps/server/src/collaboration/collaboration.util';
+import { load } from 'cheerio';
+import { normalizeImportHtml } from './apps/server/src/integrations/import/utils/import-formatter';
+import { Window } from 'happy-dom';
+
+// Mirror the exact server chain for a .md file.
+async function processMd(md: string): Promise<any> {
+  const html = await markdownToHtml(md);
+  const $ = load(html);
+  normalizeImportHtml($, $.root());
+  const normalizedHtml = $.html() || '';
+  return generateJSON(normalizedHtml, tiptapExtensions);
+}
+
+let failures = 0;
+function check(name: string, fn: () => void | Promise<void>) {
+  return Promise.resolve()
+    .then(() => fn())
+    .then(() => console.log(`✅ ${name}`))
+    .catch((err: any) => {
+      failures++;
+      console.error(`❌ ${name}: ${err?.name}: ${err?.message}`);
+      if (err?.stack) console.error(err.stack);
+    });
+}
+
+async function main() {
+  console.log('=== Section A: full import chain (markdown -> PM JSON) ===');
+  const mdCases: Array<[string, string]> = [
+    ['basic markdown', '# Title\n\n**bold** and *italic*.\n\n- a\n- b\n'],
+    ['empty', ''],
+    ['whitespace only', '   \n\n   '],
+    ['just title', '# Title'],
+    ['html entities', '# Test & <code> "quotes" </code>'],
+    ['nested lists', '# T\n\n- a\n  - b\n  - c\n- d'],
+    ['task list', '- [ ] todo\n- [x] done'],
+    ['emoji', '# Test 🎉 emoji ✓'],
+    ['cyrillic', '# Заголовок\n\nТекст на русском'],
+    ['code with special chars', '```\nconst x = "<>&"\n```'],
+    ['link', '[example](https://example.com)'],
+    ['image', '![alt](https://example.com/img.png)'],
+    ['table', '| Col1 | Col2 |\n|------|------|\n| v1 | v2 |\n'],
+    ['blockquote', '> quote\n> line2'],
+  ];
+  for (const [name, md] of mdCases) {
+    await check(`md: ${name}`, () => processMd(md));
+  }
+
+  console.log('\n=== Section B: raw generateJSON on tricky HTML fragments ===');
+  const htmlCases: Array<[string, string]> = [
+    ['plain paragraph', '<p>Hello</p>'],
+    ['deeply nested divs', '<div><div><div><p>deep</p></div></div></div>'],
+    ['unclosed-ish tags (browser-fixup)', '<b>bold<i>both</b>italic'],
+    ['empty body', ''],
+    ['only whitespace nodes', '   \n  '],
+  ];
+  for (const [name, html] of htmlCases) {
+    await check(`html: ${name}`, () => generateJSON(html, tiptapExtensions));
+  }
+
+  console.log('\n=== Section C: happy-dom cleanup behavior (generateJSON finally block) ===');
+  await check('sync finally with abort()/close() returns SUCCESS', () => {
+    // Mirrors generateJSON.ts finally exactly: no await, no try/catch.
+    const w = new Window();
+    try {
+      const dp = new w.DOMParser();
+      dp.parseFromString('<!DOCTYPE html><html><body><p>hi</p></body></html>', 'text/html');
+    } finally {
+      w.happyDOM.abort();
+      w.happyDOM.close();
+    }
+  });
+  await check('abort()/close() are Promises (async) in happy-dom 20', async () => {
+    const w = new Window();
+    const a = w.happyDOM.abort();
+    const c = w.happyDOM.close();
+    if (!(a instanceof Promise) || !(c instanceof Promise)) {
+      throw new Error('expected abort/close to return Promises');
+    }
+    await a;
+    await c;
+  });
+  await check('double close() does not throw', () => {
+    const w = new Window();
+    w.happyDOM.close();
+    w.happyDOM.close();
+  });
+
+  console.log(`\n=== Done. Failures: ${failures} ===`);
+  if (failures > 0) process.exitCode = 1;
+}
+
+main();