fix(sftp): case-insensitive inbound regex, skip _warn.txt, add targeted pull

Three changes that unblock the daily inbound pull from Gainwell's
FromHPE MFT path:

1. INBOUND_RE now accepts both 'TP' and 'tp' prefixes via inline
   (?i:TP) scoping — case-folding the whole pattern would let
   lowercase '837p' / tracking IDs through, which is invalid HCPF.
   The rest of the pattern is still case-sensitive.

2. _list_inbound_paramiko skips *_warn.txt entries. Gainwell's MFT
   drops ~583 advisory text-format notes in the same inbound dir;
   they come first alphabetically and were padding every poll with
   ~80 min of pointless downloads.

3. New SftpClient.list_inbound_names() + download_inbound() pair
   gives a metadata-only listing and on-demand fetch. The
   scheduler's existing full-listing path still works (now without
   the warn padding); the new path is what the new
   /api/admin/scheduler/pull-inbound endpoint and the
   'cyclone pull-inbound' CLI use to fast-target a date range
   without paying the cost of a full ~6000-file download.

Scheduler.process_inbound_files() runs the same per-file pipeline
as a regular tick on the pre-fetched list, so dedup via
processed_inbound_files still applies.

Tests added in test_filenames.py (lowercase + mixed-case cases),
test_sftp_paramiko.py (warn skip + no-download listing), and
test_scheduler.py (process_inbound_files idempotency).

With this, the daily 385-file pull for 20260624 completes in
seconds via 'docker exec cyclone-backend-1 python -m cyclone
pull-inbound --date 20260624 --block dzinesco' (or the equivalent
POST to /api/admin/scheduler/pull-inbound?date=20260624).

backend/tests/test_sftp_paramiko.py

@@ -259,6 +259,66 @@ class TestRealModeListInbound:
         assert len(files) == 1
         assert files[0].name == "real.x12"
 
+    def test_list_skips_warn_txt_files(self, monkeypatch, tmp_path: Path):
+        # Gainwell's MFT drops advisory *_warn.txt files in the same
+        # inbound dir. They're text-format side-channel notes (not X12
+        # envelopes) and must be skipped at list time.
+        real_attr = MagicMock()
+        real_attr.filename = "TP123-837P_M456-20260520231513488-1of1_999.x12"
+        real_attr.st_mode = 0o100644
+        real_attr.st_size = 1024
+        real_attr.st_mtime = 1718899200
+
+        warn_attr = MagicMock()
+        warn_attr.filename = "TP123-837P-202606181208100000-1of1_warn.txt"
+        warn_attr.st_mode = 0o100644
+        warn_attr.st_size = 200
+        warn_attr.st_mtime = 1718899200
+
+        mock_ssh, mock_sftp = _make_mock_paramiko(
+            monkeypatch, sftp_attrs=[real_attr, warn_attr],
+        )
+
+        def _open(path, mode="rb"):
+            m = MagicMock()
+            m.__enter__.return_value = io.BytesIO(b"x12 content")
+            return m
+
+        mock_sftp.open.side_effect = _open
+
+        block = _block(staging_dir=str(tmp_path / "staging"))
+        client = SftpClient(block)
+        files = client.list_inbound()
+
+        names = [f.name for f in files]
+        assert "TP123-837P_M456-20260520231513488-1of1_999.x12" in names
+        assert not any(n.endswith("_warn.txt") for n in names)
+        assert len(files) == 1
+
+    def test_list_inbound_names_does_not_download(self, monkeypatch, tmp_path: Path):
+        # list_inbound_names() must do a metadata-only SFTP listing —
+        # no sftp.open() / no file written to the cache.
+        attr = MagicMock()
+        attr.filename = "TP123-837P_M456-20260520231513488-1of1_999.x12"
+        attr.st_mode = 0o100644
+        attr.st_size = 1024
+        attr.st_mtime = 1718899200
+
+        mock_ssh, mock_sftp = _make_mock_paramiko(
+            monkeypatch, sftp_attrs=[attr],
+        )
+
+        block = _block(staging_dir=str(tmp_path / "staging"))
+        client = SftpClient(block)
+        files = client.list_inbound_names()
+
+        # sftp.open() must NOT have been called (no download).
+        mock_sftp.open.assert_not_called()
+        assert len(files) == 1
+        # The InboundFile.local_path is set to the planned cache path
+        # but the file itself doesn't exist yet.
+        assert not files[0].local_path.exists()
+
 
 class TestRealModeReadFile:
     def test_read_returns_bytes(self, monkeypatch):