fix(review): never queue an ambiguous file with nothing to choose

The maintainer's live queue held 39 items whose candidates all read
"unresolved" ("No positive / No negative"). An ambiguous result is now
reviewable only when at least two candidates are resolved with non-blank
positive text. With none the file could not be traced; with exactly one,
the untraced branches may still have made the file, so that text is not
certain either. Both are reported unresolved, never linked and never
queued (also the A1111 two-delimiter case, which has no candidates).

A rescan removes the pending items of such files whose own evidence
offers no choice, with the same safe delete as the linked settle
(pending, revision 0, no decision row). A limited item (over 16 candidates) that holds at least two
candidates is kept. The job scope gains review_untraced and the log
"= N images left Needs review · nothing to choose".

tests/test_api_admin.py

@@ -2223,7 +2223,9 @@ class TestActivityJobs(AdminAPITestCase):
         for name in names:
             (self.output_dir / name).write_bytes(b"not an image")
 
-        def fake_ingest(media_file, metadata, detailed=False, ambiguous=None):
+        def fake_ingest(
+            media_file, metadata, detailed=False, ambiguous=None, untraced=None
+        ):
             key = Path(media_file).relative_to(self.output_dir).as_posix()
             outcome = names[key]
             if isinstance(outcome, Exception):