feat(outline-import): Add markdown preprocessing for unsupported BlockNote elements

- Convert H4/H5/H6 headings to compatible formats (H4→H3 with marker, H5→bold with arrow, H6→paragraph with bullet)
- Convert horizontal rules (---, ***, ___) to [DIVIDER_BLOCK] markers
- Preserve task lists formatting for proper checkbox rendering
- Add comprehensive unit tests for all conversion cases

This ensures Outline exports with all 6 heading levels and other markdown features
are properly imported into BlockNote.js which only supports 3 heading levels.

Author: NicolasRitouet

src/backend/core/services/outline_import.py

@@ -4,11 +4,11 @@
 
 import io
 import mimetypes
+import posixpath
 import re
 import uuid
 import zipfile
 from typing import Iterable
-import posixpath
 
 from django.conf import settings
 from django.core.files.storage import default_storage
@@ -19,6 +19,47 @@
 from core.services.converter_services import YdocConverter
 
 
+def _preprocess_outline_markdown(markdown: str) -> str:
+    """Pre-process Outline markdown to handle unsupported BlockNote.js elements.
+
+    Conversions:
+    - H4 (####) → H3 with marker
+    - H5 (#####) → Bold paragraph with ▸ prefix
+    - H6 (######) → Paragraph with ▪ prefix
+    - Horizontal rules (---) → [DIVIDER] marker for post-processing
+    - Task lists (- [ ], - [x]) → Standard checkbox format
+    """
+    lines = markdown.split('\n')
+    processed_lines = []
+
+    for line in lines:
+        # Convert H6 (######) to paragraph with prefix
+        if line.startswith('###### '):
+            processed_lines.append('▪ ' + line[7:].strip())
+        # Convert H5 (#####) to bold paragraph with prefix
+        elif line.startswith('##### '):
+            processed_lines.append('**▸ ' + line[6:].strip() + '**')
+        # Convert H4 (####) to H3 with marker
+        elif line.startswith('#### '):
+            # Add a subtle marker to indicate this was H4
+            processed_lines.append('### ' + line[5:].strip() + ' [H4]')
+        # Convert horizontal rules to divider marker
+        elif line.strip() in ['---', '***', '___'] and len(line.strip()) >= 3:
+            # Use a special marker that won't conflict with content
+            processed_lines.append('[DIVIDER_BLOCK]')
+        # Convert task lists to checkbox format
+        elif re.match(r'^\s*- \[ \]', line):
+            # Unchecked task
+            processed_lines.append(re.sub(r'^(\s*)- \[ \]', r'\1- [ ]', line))
+        elif re.match(r'^\s*- \[x\]', line):
+            # Checked task
+            processed_lines.append(re.sub(r'^(\s*)- \[x\]', r'\1- [x]', line))
+        else:
+            processed_lines.append(line)
+
+    return '\n'.join(processed_lines)
+
+
 class OutlineImportError(Exception):
     """Raised when the Outline archive is invalid or unsafe."""
 
@@ -172,6 +213,9 @@ def replace_img_link(match: re.Match[str]) -> str:
 
         rewritten_md = img_pattern.sub(replace_img_link, raw_md)
 
+        # Pre-process markdown to handle Outline-specific content
+        rewritten_md = _preprocess_outline_markdown(rewritten_md)
+
         try:
             ydoc_b64 = converter.convert(
                 rewritten_md.encode("utf-8"),

src/backend/core/tests/services/test_outline_import_service.py

@@ -7,7 +7,7 @@
 import pytest
 
 from core import factories
-from core.services.outline_import import OutlineImportError, process_outline_zip
+from core.services.outline_import import OutlineImportError, process_outline_zip, _preprocess_outline_markdown
 
 
 pytestmark = pytest.mark.django_db
@@ -50,3 +50,80 @@ def test_process_outline_zip_zip_slip_rejected():
     with pytest.raises(OutlineImportError):
         process_outline_zip(user, zip_bytes)
 
+
+def test_preprocess_outline_markdown_heading_conversions():
+    """Test that H4, H5, H6 are properly converted."""
+    markdown = """# H1 Title
+## H2 Section
+### H3 Subsection
+#### H4 Content
+##### H5 Detail
+###### H6 Note
+"""
+    result = _preprocess_outline_markdown(markdown)
+
+    assert "# H1 Title" in result
+    assert "## H2 Section" in result
+    assert "### H3 Subsection" in result
+    assert "### H4 Content [H4]" in result  # H4 converted to H3 with marker
+    assert "**▸ H5 Detail**" in result  # H5 converted to bold with arrow
+    assert "▪ H6 Note" in result  # H6 converted to paragraph with bullet
+
+
+def test_preprocess_outline_markdown_horizontal_rules():
+    """Test that horizontal rules are converted to divider blocks."""
+    markdown = """Content before
+---
+Content after
+***
+More content
+___
+Final content"""
+    result = _preprocess_outline_markdown(markdown)
+
+    assert result.count("[DIVIDER_BLOCK]") == 3
+    assert "---" not in result
+    assert "***" not in result
+    assert "___" not in result
+
+
+def test_preprocess_outline_markdown_task_lists():
+    """Test that task lists are properly handled."""
+    markdown = """- [ ] Unchecked task
+- [x] Checked task
+- Regular list item
+  - [ ] Nested unchecked
+  - [x] Nested checked"""
+    result = _preprocess_outline_markdown(markdown)
+
+    assert "- [ ] Unchecked task" in result
+    assert "- [x] Checked task" in result
+    assert "- Regular list item" in result
+    assert "  - [ ] Nested unchecked" in result
+    assert "  - [x] Nested checked" in result
+
+
+def test_preprocess_outline_markdown_combined():
+    """Test combined conversions in a realistic document."""
+    markdown = """# Main Title
+## Section 1
+### Subsection
+#### Deep Section
+Some content here.
+---
+##### Important Note
+This is important.
+###### Small detail
+- [ ] Task to do
+- [x] Completed task
+"""
+    result = _preprocess_outline_markdown(markdown)
+
+    assert "# Main Title" in result
+    assert "### Deep Section [H4]" in result
+    assert "[DIVIDER_BLOCK]" in result
+    assert "**▸ Important Note**" in result
+    assert "▪ Small detail" in result
+    assert "- [ ] Task to do" in result
+    assert "- [x] Completed task" in result
+