perf(converter): lazy-import markitdown to cut startup and enable slim packaging

markitdown is only used for the non-PDF, non-Markdown ingest branch, but it
was imported at module top level — so every `import openkb` eagerly pulled in
markitdown → magika → onnxruntime (tens of MB, slow). Move the import into the
branch that actually uses it.

Effects (measured):
- `import openkb.cli` cold start 4.64s → 3.53s (~24% faster); magika /
  onnxruntime / markitdown no longer load unless an Office-format document is
  converted.
- A packaged (PyInstaller) build can now exclude magika/onnxruntime for a
  slimmer artifact when Office-format ingest isn't needed. Previously the
  unconditional module-level markitdown import made magika load-bearing at
  startup, so excluding it crashed the whole app on launch.

markitdown appends warning filters on import but leaves the CLI's front
"ignore" filter at index 0, so no re-suppression is needed at the new import
site. test_converter patches `markitdown.MarkItDown` (the real attribute the
local import binds) instead of the removed `openkb.converter.MarkItDown`.

Claude-Session: https://claude.ai/code/session_01KZyUSGAzVL9yxpsWWPv6Y2

Author: claude

openkb/converter.py

@@ -11,7 +11,6 @@
 from pathlib import Path
 
 import pymupdf
-from markitdown import MarkItDown
 
 from openkb.config import load_config
 from openkb.images import convert_pdf_with_images, copy_relative_images, extract_base64_images
@@ -230,7 +229,15 @@ def convert_document(
             # Use pymupdf dict-mode for PDFs: text + images inline at correct positions
             markdown = convert_pdf_with_images(src, doc_name, images_dir)
         else:
-            # Non-PDF, non-MD: use markitdown (docx, pptx, html, etc.)
+            # Non-PDF, non-MD: use markitdown (docx, pptx, html, etc.).
+            # Imported lazily: markitdown pulls in magika → onnxruntime (tens
+            # of MB, slow to import) that only this branch needs. Keeping it
+            # out of module import makes `import openkb` cheap and lets a
+            # packaged build drop those deps when Office-format ingest isn't
+            # required. markitdown appends warning filters on import but leaves
+            # the CLI's front "ignore" filter in place, so no re-suppress here.
+            from markitdown import MarkItDown
+
             mid = MarkItDown()
             result = mid.convert(str(src), keep_data_uris=True)
             markdown = result.text_content

tests/test_converter.py

@@ -145,7 +145,7 @@ def test_docx_conversion_enables_keep_data_uris(self, kb_dir, tmp_path):
         mock_result.text_content = "![](data:image/png;base64,abc123)"
 
         with (
-            patch("openkb.converter.MarkItDown") as mock_markitdown,
+            patch("markitdown.MarkItDown") as mock_markitdown,
             patch(
                 "openkb.converter.extract_base64_images",
                 return_value="converted markdown",