polish(mail): consistency guards (PR #935 round-7)

Non-blocking consistency fixes:
- scanner.py: skip re-queuing a mail_message whose placeholder status is
  "failed" (mirrors the file scanner's permanent-failure guard); the
  modified_at branch still retries once the message changes.
- search/context.py: return None on an empty get_message payload during context
  expansion, mirroring the processor's index-time empty-payload guard.
- tests: cover the non-numeric mailbox_id fail-open branch in the verifier.

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

tests/unit/search/test_verification.py

@@ -303,6 +303,22 @@ async def test_verify_mail_missing_mailbox_id_keeps(mocker):
     list_messages.assert_not_awaited()
 
 
+@pytest.mark.unit
+async def test_verify_mail_non_numeric_mailbox_id_keeps(mocker):
+    """A non-numeric mailbox_id in metadata is kept without a list call."""
+    list_messages = mocker.AsyncMock()
+    mail_client = SimpleNamespace(list_messages=list_messages)
+    client = SimpleNamespace(mail=mail_client, username="alice")
+
+    result = await _verify_mail_messages(
+        client,
+        [_make_result(42, doc_type="mail_message", metadata={"mailbox_id": "bad"})],
+        _sem(),
+    )
+    assert result == {"42"}
+    list_messages.assert_not_awaited()
+
+
 @pytest.mark.unit
 async def test_verify_mail_non_numeric_id_kept_when_mailbox_listed(mocker):
     """A malformed doc_id can't match the numeric listing, so it's kept."""