Fix: index document versions as their root document

The search index and the LLM index hold root documents only, a root being
indexed with its newest version's content. Every write path therefore had to
remember to hand them the root. Several did not: adding or deleting a note on
a version, restoring a trashed document together with its versions, and
reprocessing a version all wrote the version into the index under its own id,
where it could be returned as a separate search result. The paths that walk the
whole library did the same: the document_index reindex command put every
version in the search index, a full LLM index rebuild embedded every version,
and an incremental LLM update scoped to a version id indexed it under its own
id. Asking for documents like a version looked the version's own id up in the
index and silently found nothing.

WriteBatch.add_or_update, WriteBatch.add_or_update_ids and
llm_index_add_or_update_document now resolve a version to its root themselves.
The reindex command and update_llm_index only walk root documents, and an
incremental LLM update is scoped to the roots of the given ids through
versioning.root_document_ids, which add_or_update_ids shares. more_like_id
returns the root's id for a version. The reprocess task no longer picks the root
for the indexes and only still clears the caches of both documents.

src/documents/versioning.py

@@ -17,9 +17,26 @@ from django.db.models.functions import RowNumber
 from documents.models import Document
 
 if TYPE_CHECKING:
+    from collections.abc import Iterable
+
     from rest_framework.request import Request
 
 
+def root_document_ids(ids: Iterable[int]) -> QuerySet[int]:
+    """
+    The ids of the root documents of the given documents: a root stands for
+    itself and a version for its root. Only the indexes' bookkeeping needs
+    this, since they hold root documents only.
+    """
+    return (
+        Document.objects.filter(pk__in=ids)
+        .annotate(root_id=Coalesce("root_document_id", "id"))
+        .order_by()
+        .values_list("root_id", flat=True)
+        .distinct()
+    )
+
+
 def versions_newest_first(documents: QuerySet[Document]) -> QuerySet[Document]:
     """
     Sorts versions so the newest one comes first using version_index and not on id,