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/core/page/services/page.service.ts

@@ -52,7 +52,9 @@ import {
   INTERNAL_LINK_REGEX,
   extractPageSlugId,
 } from '../../../integrations/export/utils';
-import { markdownToHtml, canonicalizeFootnotes } from '@docmost/editor-ext';
+import { canonicalizeFootnotes } from '@docmost/editor-ext';
+import { markdownToProseMirror } from '@docmost/prosemirror-markdown';
+import { normalizeForeignMarkdown } from '../../../integrations/import/utils/foreign-markdown';
 import { WatcherService } from '../../watcher/watcher.service';
 import { sql } from 'kysely';
 import { TransclusionService } from '../transclusion/transclusion.service';
@@ -1301,8 +1303,14 @@ export class PageService {
 
     switch (format) {
       case 'markdown': {
-        const html = await markdownToHtml(content as string);
-        prosemirrorJson = htmlToJson(html as string);
+        // Canonical markdown -> ProseMirror JSON directly via
+        // `@docmost/prosemirror-markdown` (issue #345) — no HTML intermediate,
+        // no editor-ext markdown layer. Foreign markdown surfaces the strict
+        // parser rejects (GFM `[^id]` reference footnotes) are normalized to the
+        // canonical inline form first.
+        prosemirrorJson = await markdownToProseMirror(
+          normalizeForeignMarkdown(content as string),
+        );
         break;
       }
       case 'html': {