Performance: batch permission assignment in bulk set_permissions (#13806)

* Perf: batch guardian permission assignment in bulk-edit

bulk_edit.set_permissions and BulkEditObjectPermissionsView both
looped documents/objects and called set_permissions_for_object per
object, which itself calls guardian's assign_perm/remove_perm once
per (object, user) pair -- ~10-20+ queries per object, scaling with
selection size.

Added set_permissions_for_objects, a bulk equivalent that resolves
existing permission holders once across the whole batch (not once per
object) and applies changes with a small, batch-size-independent
number of queries per action instead of one per (object, user) pair.

* Perf: avoid unnecessary full-row fetches in batch permission assignment

set_permissions_for_objects now takes a model + pks instead of instances,
and identity filtering resolves straight to ids, so bulk-editing
permissions no longer materializes full Document/User/Group rows just to
read their pk/id. Row construction for bulk_create is also chunked to
bound peak memory for very large "apply to all" operations.

* Fix: use .distinct() for existing-grant lookup, drop flaky query-count invariant tests

.distinct() lets the database dedupe identity ids server-side instead of
transferring one row per (object, grantee) match and deduping in Python --
was the dominant cost on a large selection with existing grants.

Also replaced the two query-count-equality tests (bulk_edit and the
bulk_edit_objects API path) with plain functional-correctness checks at
both batch sizes.  Hopefully stops that flake.

* Mark empty-pks early-return in set_permissions_for_objects as no-cover

Defensive guard for an edge case (all requested pks already gone/invalid)
rather than a path normal usage exercises; matches the existing
pragma: no cover convention elsewhere in this file.

* Perf: drop speculative row-chunking in bulk permission assignment, keep the query-batching fix

* Resolve every permission action before applying any of them

This fixes the existing issue and resolves the Copilot comment

* Assert permission assignment does not scale with selection size

The two batching tests only checked that permissions came out correct at
5 and 50 objects, so reverting to the old per-object loop would still
have passed.  Check sizes as well to prevent that

src/documents/bulk_edit.py

@@ -28,7 +28,7 @@ from documents.models import DocumentType
 from documents.models import PaperlessTask
 from documents.models import StoragePath
 from documents.models import Tag
-from documents.permissions import set_permissions_for_object
+from documents.permissions import set_permissions_for_objects
 from documents.plugins.helpers import DocumentsStatusManager
 from documents.tasks import bulk_update_documents
 from documents.tasks import consume_file
@@ -433,10 +433,13 @@ def set_permissions(
     else:
         qs.update(owner=owner)
 
-    for doc in qs:
-        set_permissions_for_object(permissions=set_permissions, object=doc, merge=merge)
-
     affected_docs = list(qs.values_list("pk", flat=True))
+    set_permissions_for_objects(
+        permissions=set_permissions,
+        model=Document,
+        pks=affected_docs,
+        merge=merge,
+    )
 
     bulk_update_documents.apply_async(
         kwargs={"document_ids": affected_docs},