refactor(#345 step 2): server markdown IMPORT via canonical parser + normalizer

Move every SERVER Markdown->ProseMirror path off the editor-ext markdown layer
(`markdownToHtml`, a second marked-based parser) onto the canonical
`@docmost/prosemirror-markdown` package, and add a foreign-markdown normalizer at
the import boundary.

Code:
- `ImportService.processMarkdown` (single `.md` upload) now parses
  `markdownToProseMirror(normalizeForeignMarkdown(md))` directly — no HTML hop.
- `PageService.parseProsemirrorContent` markdown case (page create/update with
  `format: 'markdown'`) same.
- `FileImportTaskService` (zip import) parses markdown with the package, then
  serializes to HTML (`jsonToHtml`) so the SHARED HTML attachment / internal-link
  pipeline (processAttachments + formatImportHtml + processHTML) keeps handling
  `.md` and `.html` imports uniformly. The markdown PARSE — the drift source — no
  longer goes through editor-ext; the PM->HTML->PM hop that follows is lossless
  plumbing for attachment resolution, not a second parse.
- `canonicalizeFootnotes` stays as an idempotent #228 safety net for the HTML
  path (a no-op on the already-canonical markdown output).

Normalizer (`integrations/import/utils/foreign-markdown.ts`): a TEXT pre-pass,
NOT a parser fork. The strict canonical parser does not accept GFM `[^id]`
reference footnotes (and would misread `[^id]: def` as a CommonMark link-ref
definition, silently corrupting the ref into a bogus link), so the normalizer
rewrites reference footnotes into canonical inline `^[def]` before parsing.
Callout surfaces (`:::type` and `> [!type]`) are intentionally NOT touched — the
canonical parser already accepts BOTH natively, so normalizing them would be
redundant and risk degrading its nesting/code-fence-aware handling.

Fixtures-first: foreign-markdown.spec pins the normalizer and the end-to-end
acceptance (no literal `[^id]`/`:::` leaks; re-export is canonical). The two
footnote-canonicalize specs are updated to the canonical output — the parser
assigns fresh `fn-*` ids, so they now assert by definition BODY order (still
reference-ordered, deduped, orphan-free).

FINAL CHECK: `grep -rn "htmlToMarkdown\|markdownToHtml" apps/server/src` (non
-test) is now empty — both editor-ext markdown-layer functions are gone from the
server.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/integrations/import/utils/foreign-markdown.spec.ts

@@ -0,0 +1,113 @@
+import {
+  convertProseMirrorToMarkdown,
+  markdownToProseMirror,
+} from '@docmost/prosemirror-markdown';
+import { normalizeForeignMarkdown } from './foreign-markdown';
+
+/**
+ * STEP 2 goldens for issue #345: the foreign-markdown normalizer that runs at the
+ * import boundary BEFORE the strict canonical parser (`markdownToProseMirror`).
+ *
+ * Two layers:
+ *  1. PURE string→string cases pinning the normalizer's own behavior (GFM
+ *     reference footnotes → inline `^[…]`).
+ *  2. END-TO-END acceptance: for a foreign corpus, `normalizeForeignMarkdown`
+ *     then `markdownToProseMirror` then `convertProseMirrorToMarkdown` must leave
+ *     NO literal `[^id]` / `:::` garbage in the document and must re-export in the
+ *     canonical forms.
+ */
+
+describe('normalizeForeignMarkdown — GFM reference footnotes', () => {
+  it('inlines a single-line reference footnote and drops its definition', () => {
+    const out = normalizeForeignMarkdown(
+      'A note[^1] here.\n\n[^1]: The definition.',
+    );
+    expect(out).toBe('A note^[The definition.] here.\n');
+  });
+
+  it('inlines every reference to a reused id (downstream dedups)', () => {
+    const out = normalizeForeignMarkdown(
+      'X[^a] and Y[^a].\n\n[^a]: shared.',
+    );
+    expect(out).toBe('X^[shared.] and Y^[shared.].\n');
+  });
+
+  it('joins indented continuation lines of a definition with a space', () => {
+    const out = normalizeForeignMarkdown(
+      'See[^n].\n\n[^n]: line one\n    line two',
+    );
+    expect(out).toBe('See^[line one line two].\n');
+  });
+
+  it('never rewrites a reference inside a fenced code block', () => {
+    const out = normalizeForeignMarkdown(
+      '```\ncode[^1] here\n```\n\n[^1]: def.',
+    );
+    expect(out).toContain('code[^1] here');
+    // The (now orphaned) definition line is still removed.
+    expect(out).not.toContain('[^1]: def.');
+  });
+
+  it('leaves a reference with no matching definition literal (no body to inline)', () => {
+    const out = normalizeForeignMarkdown('Dangling[^x] ref.');
+    expect(out).toBe('Dangling[^x] ref.');
+  });
+
+  it('returns the input unchanged when there are no reference footnotes', () => {
+    const md = '# Title\n\nJust text with `inline code` and a [link](/x).';
+    expect(normalizeForeignMarkdown(md)).toBe(md);
+  });
+
+  it('does NOT touch callout surfaces — the canonical parser handles them', () => {
+    const callouts = ':::info\nHi\n:::\n\n> [!warning]\n> Careful';
+    expect(normalizeForeignMarkdown(callouts)).toBe(callouts);
+  });
+});
+
+describe('foreign markdown import acceptance (normalizer + canonical parser)', () => {
+  const FOREIGN = [
+    '# Doc',
+    '',
+    'Body refs [^c] and [^a] and [^b] and again [^a].',
+    '',
+    ':::info',
+    'A legacy callout.',
+    ':::',
+    '',
+    '| h1 | h2 |',
+    '| --- | --- |',
+    '| 1 | 2 |',
+    '',
+    '[^a]: note A',
+    '[^b]: note B',
+    '[^c]: note C',
+    '[^z]: orphan note',
+  ].join('\n');
+
+  it('leaves no literal [^id] or ::: in the imported doc and re-exports canonically', async () => {
+    const normalized = normalizeForeignMarkdown(FOREIGN);
+    const doc = await markdownToProseMirror(normalized);
+    const reexport = convertProseMirrorToMarkdown(doc);
+
+    // No foreign garbage leaks into the document.
+    expect(reexport).not.toMatch(/\[\^/); // no reference footnote refs/defs
+    expect(reexport).not.toContain(':::'); // no legacy callout fences
+
+    // Canonical forms are present.
+    expect(reexport).toContain('^[note C]');
+    expect(reexport).toContain('> [!info]');
+    expect(reexport).toContain('| h1 | h2 |');
+
+    // Footnotes: ordered by first reference (C, A, B), reused [^a] deduped to one,
+    // orphan [^z] dropped (it had no reference after normalization).
+    const list = doc.content.find((n: any) => n.type === 'footnotesList');
+    const bodies = list.content.map(
+      (d: any) => d.content[0].content[0].text,
+    );
+    expect(bodies).toEqual(['note C', 'note A', 'note B']);
+    expect(bodies).not.toContain('orphan note');
+    expect(
+      doc.content.filter((n: any) => n.type === 'footnotesList'),
+    ).toHaveLength(1);
+  });
+});