feat(sp16): live MFT polling scheduler

Adds an asyncio-based background scheduler that polls the Gainwell
MFT inbound path, downloads new files, and routes them through the
appropriate parser (999 / 835 / 277CA / TA1). Idempotent (re-ticks
and restarts skip already-processed files via the new
processed_inbound_files table). Crash-safe (per-file try/except so
one bad file doesn't stop the loop).

Lifespan auto-configures from the seeded dzinesco clearhouse's SFTP
block; auto-start is opt-in via CYCLONE_SCHEDULER_AUTOSTART.

Five admin endpoints added:
  GET  /api/admin/scheduler/status
  POST /api/admin/scheduler/start
  POST /api/admin/scheduler/stop
  POST /api/admin/scheduler/tick
  GET  /api/admin/scheduler/processed-files?status=&limit=

20 new tests (15 unit + 5 API).

backend/tests/test_sftp_stub.py

@@ -72,7 +72,23 @@ def test_stub_get_secret_returns_stub_when_keychain_empty(sftp_block):
     assert secret == "<stub-secret>"
 
 
-def test_stub_read_file_raises(sftp_block):
+def test_stub_read_file_returns_bytes(sftp_block, tmp_path):
+    """SP16: the stub read_file returns bytes from the staging dir.
+
+    Lets the inbound scheduler exercise the same code path on a
+    workstation without a real MFT connection.
+    """
+    inbound = tmp_path / "staging" / "ToHPE"
+    inbound.mkdir(parents=True)
+    (inbound / "TP11525703-837P_M019048402-20260618130000000-1of1_TA1.x12").write_bytes(
+        b"hello-world",
+    )
     client = SftpClient(sftp_block)
-    with pytest.raises(RuntimeError, match="Stub SFTP cannot read"):
-        client.read_file("/x.x12")
+    body = client.read_file("/ToHPE/TP11525703-837P_M019048402-20260618130000000-1of1_TA1.x12")
+    assert body == b"hello-world"
+
+
+def test_stub_read_file_missing_raises(sftp_block):
+    client = SftpClient(sftp_block)
+    with pytest.raises(FileNotFoundError):
+        client.read_file("/ToHPE/does-not-exist.x12")