fix(sp37): submit_file default factory uses paramiko directly

SftpClient wrapper lacks a stat() method, so the previous default
factory produced a client whose stat() raised AttributeError — making
the SKIPPED outcome unreachable in production and silently
misclassifying all uploads as SFTP_FAILED. The helper now opens a
paramiko SFTP session directly (same pattern as
resubmit-rejected-claims in cli.py:620-650), so stat() and write_file()
both work end-to-end.

Also: tighten typing (SftpBlock instead of Any), add PAYER_MISMATCH
test, drop duplicate _db fixture, rename misleading test, add class
docstrings to SubmitResult/SubmitOutcome.

backend/tests/test_submission.py

@@ -1,8 +1,8 @@
 """SP37 Task 4: submit_file owns parse → DB write → SFTP upload.
 
 Tests use a fake SFTP client to avoid hitting the real clearhouse.
-The DB path is real (autouse fixture) so the join-key wiring is
-exercised end-to-end.
+The DB path is real (autouse ``_auto_init_db`` fixture in conftest.py)
+so the join-key wiring is exercised end-to-end.
 """
 from __future__ import annotations
 
@@ -12,7 +12,6 @@ from unittest.mock import MagicMock
 import pytest
 
 from cyclone import db as db_mod
-from cyclone.clearhouse import SftpClient
 from cyclone.submission.core import submit_file
 from cyclone.submission.result import SubmitOutcome
 
@@ -48,14 +47,6 @@ class _FakeSftp:
         self.existing_files[remote_path] = len(content)
 
 
-@pytest.fixture(autouse=True)
-def _db(tmp_path, monkeypatch):
-    monkeypatch.setenv("CYCLONE_DB_URL", f"sqlite:///{tmp_path}/test.db")
-    db_mod._reset_for_tests()
-    db_mod.init_db()
-    yield
-
-
 def test_submit_file_happy_path():
     """parse → DB write → SFTP upload, all steps succeed."""
     fake = _FakeSftp()
@@ -79,8 +70,14 @@ def test_submit_file_happy_path():
         assert s.get(db_mod.Batch, result.batch_id) is not None
 
 
-def test_submit_file_idempotent_db():
-    """Re-running submit_file on the same file is a no-op on the DB side."""
+def test_submit_file_skipped_when_remote_size_matches():
+    """Re-running submit_file on an already-uploaded file is SKIPPED.
+
+    The ``SKIPPED`` outcome is reached via the SFTP ``stat()`` check
+    (not via the DB) — same-size remote file ⇒ idempotency short-circuit.
+    Re-runs therefore don't emit a duplicate ``clearhouse.submitted``
+    audit event.
+    """
     fake = _FakeSftp()
     sftp_block = MagicMock(stub=False, paths={"outbound": "/ToHPE"})
     factory = lambda _b: fake
@@ -137,4 +134,62 @@ def test_submit_file_sftp_fail():
     # DB row was written before the SFTP call.
     assert result.batch_id is not None
     with db_mod.SessionLocal()() as s:
-        assert s.get(db_mod.Batch, result.batch_id) is not None
+        assert s.get(db_mod.Batch, result.batch_id) is not None
+
+
+def test_submit_file_payer_mismatch():
+    """PAYER_MISMATCH → no DB write, no SFTP call.
+
+    We monkeypatch ``parse_837_text`` to return a single claim whose
+    payer.id is ``OTHER_PAYER`` (the helper gates on the first
+    claim's payer — same posture as the legacy ``resubmit_rejected_claims``
+    CLI at cli.py:657-668).
+    """
+    from unittest.mock import patch
+
+    bad_claim = MagicMock()
+    bad_claim.payer.id = "OTHER_PAYER"
+    parsed_with_bad_payer = MagicMock()
+    parsed_with_bad_payer.claims = [bad_claim]
+    parsed_with_bad_payer.parsed_at = None
+    parsed_with_bad_payer.envelope.transaction_set_control_number = "ST123"
+
+    fake = _FakeSftp()
+    sftp_block = MagicMock(stub=False, paths={"outbound": "/ToHPE"})
+
+    with patch("cyclone.submission.core.parse_837_text", return_value=parsed_with_bad_payer):
+        result = submit_file(
+            _FIXTURE,
+            sftp_block=sftp_block,
+            actor="t",
+            validate=True,
+            sftp_client_factory=lambda _b: fake,
+        )
+    assert result.outcome == SubmitOutcome.PAYER_MISMATCH
+    assert "OTHER_PAYER" in (result.error or "")
+    assert len(fake.put_calls) == 0
+
+
+def test_submit_file_uses_default_paramiko_factory(monkeypatch):
+    """When no factory is supplied, helper should use paramiko directly
+    (not SftpClient wrapper, which lacks stat()).
+    """
+    fake_paramiko_sftp = _FakeSftp()
+    monkeypatch.setattr(
+        "cyclone.submission.core._default_sftp_factory",
+        lambda _block: fake_paramiko_sftp,
+    )
+
+    sftp_block = MagicMock()
+    sftp_block.stub = False
+    sftp_block.paths = {"outbound": "/ToHPE"}
+
+    result = submit_file(
+        _FIXTURE,
+        sftp_block=sftp_block,
+        actor="t",
+        validate=True,
+        # NO sftp_client_factory — should use the paramiko default
+    )
+    assert result.outcome == SubmitOutcome.SUBMITTED
+    assert len(fake_paramiko_sftp.put_calls) == 1