fix: populate chunk.doc_items on rechunk so bbox↔chunk linking works

After clicking a chunk card in the Chunk view, the matching bboxes
were not highlighted on the preview. Root cause: the rechunk path
in `ChunkService.rechunk_document` created the new `Chunk` rows
with `doc_items=[]`, on a stale comment that claimed
`ChunkResult` had no `doc_items`.

`ChunkResult` actually carries `doc_items: list[ChunkDocItem]`
(see `domain/value_objects.py`), populated by `LocalChunker` from
`chunk.meta.doc_items`. Dropping that list at rechunk time meant
the frontend's `elementRefsForChunk` helper had no `self_ref`
entries to drive the canvas highlight (#264), so the bbox stayed
in its default style regardless of the selected chunk.

Fix: pass `list(r.doc_items)` through.

Affects every chunk produced via the Strategy popover (#268) and
every chunk produced by the workspace-level "Generate chunks"
button (#266) — both go through `rechunk_document`. The
promote-from-analysis path already populated `doc_items` correctly
(see `_analysis_chunk_to_canonical`).

48 chunk-service / API tests passing.

Author: pjmalandrino

document-parser/services/chunk_service.py

@@ -451,7 +451,7 @@ async def rechunk_document(self, document_id: str, options: dict | None = None)
                 headings=list(r.headings),
                 source_page=r.source_page,
                 bboxes=list(r.bboxes),
-                doc_items=[],  # ChunkResult has no doc_items currently
+                doc_items=list(r.doc_items),
                 token_count=r.token_count or None,
             )
             for seq, r in enumerate(new_results)