fix: address PR #813 latest review (ACL-aware doc-type discovery, robustness)

- get_indexed_doc_types: add optional accessible_owners param and reuse
  build_ownership_filter so cross-user doc-type discovery matches the real
  search scope (was self-only / ACL-blind); docstring documents the self-only
  default. Covered by test_get_indexed_doc_types_is_acl_aware.
- access_filter: build_ownership_filter now omits the owner_id branch entirely
  for an empty owner set instead of relying on undocumented MatchAny(any=[])
  semantics; updated the empty-list unit test accordingly.
- access_filter: make the uid_owner/owner share-owner extraction explicit
  ("absent, not empty") to avoid skipping on a falsy-but-present field.
- access_filter: add an operator note that pre-owner_id points need a re-index
  to surface to share recipients (ACL search is a no-op for legacy data).
- verification/webdav: lock the file_accessible_by_id(scope="") contract with a
  targeted multi-user test (owner + recipient True, non-recipient False).
- viz_routes: comment that verify-on-read eviction runs inline by design (no
  lifespan task group available on the Starlette route).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/search/test_access_filter.py

@@ -174,13 +174,15 @@ class TestBuildOwnershipFilter:
         # legacy match path, so it must NOT widen to other owners.
         assert user_branch.match.value == "alice"
 
-    def test_explicit_empty_list_still_keeps_legacy_branch(self) -> None:
-        # Edge case: caller passed an explicit empty list. We shouldn't
-        # silently re-default to [user_id] in the owner branch, but the
-        # legacy branch is still the safety net so the user can find their
-        # own content from before the migration.
+    def test_explicit_empty_list_omits_owner_branch_keeps_legacy(self) -> None:
+        # Edge case: caller passed an explicit empty list. The owner_id branch
+        # is omitted entirely (rather than relying on MatchAny(any=[]) matching
+        # nothing); the legacy user_id branch remains as the safety net so the
+        # user still finds their own content from before the migration.
         flt = build_ownership_filter("alice", [])
 
-        owner_branch, user_branch = flt.should
-        assert owner_branch.match.any == []
+        assert flt.should is not None
+        assert len(flt.should) == 1
+        (user_branch,) = flt.should
+        assert user_branch.key == "user_id"
         assert user_branch.match.value == "alice"