perf(search): skip exclusion lookup on empty tag set; fix semaphore comment

Address the two important findings from the claude bot's latest re-review:

- _verify_files: skip get_excluded_file_paths entirely when the tag REPORT
  returns no files. An empty `tagged` yields an empty `tagged_ids` regardless
  of exclusions, so the lookup's 2xlen(EXCLUDED_TAGS) WebDAV fan-out is wasted
  work in the common "this tag matched nothing" case. The per-result loop still
  runs, so malformed doc_ids are still kept (fail-open) — pinned by a new test
  (test_verify_files_empty_tag_set_skips_exclusion_lookup), which also asserts
  the exclusion lookup is never awaited.
- Rewrite the semaphore comment: it claimed "the slot bounds them", but the slot
  only caps concurrent *searches* — get_excluded_file_paths internally spawns a
  task group issuing 2xlen(EXCLUDED_TAGS) concurrent WebDAV calls, so live
  Nextcloud connections can exceed VERIFICATION_CONCURRENCY. Comment now says so
  and points at configuration.md.

The third 🟡 (sequential dir expansion in find_files_by_tag) is pre-existing and
flagged by the reviewer as a follow-up, not part of this PR.

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

tests/unit/search/test_verification.py

@@ -504,6 +504,30 @@ async def test_verify_files_deleted_drops(mocker):
     assert result == set()
 
 
+@pytest.mark.unit
+async def test_verify_files_empty_tag_set_skips_exclusion_lookup(mocker):
+    """When the tag REPORT returns no files, the EXCLUDED_TAGS lookup is skipped
+    entirely: an empty tagged set drops every valid-id result regardless of
+    exclusions, so the lookup's 2xN WebDAV fan-out is wasted work. Malformed
+    doc_ids are still kept (fail-open), exactly as on the non-empty path."""
+    excluded = _patch_excluded(mocker, {"Secret"})
+    client = _file_client(mocker, tagged=[])
+
+    result = await _verify_files(
+        client,
+        [
+            _make_result(123, doc_type="file"),
+            _make_result("not-a-file-id", doc_type="file"),
+        ],
+        _sem(),
+    )
+
+    # Valid id absent from the (empty) tagged set → dropped; malformed id kept.
+    assert result == {"not-a-file-id"}
+    # The optimization: no exclusion fan-out when there is nothing to filter.
+    excluded.assert_not_awaited()
+
+
 @pytest.mark.unit
 async def test_verify_files_excluded_path_drops(mocker):
     """A tagged file under an EXCLUDED_TAGS folder must not surface — exclusion