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_acks.py

@@ -51,18 +51,19 @@ def test_migration_0002_creates_acks_table():
 
 def test_migration_latest_idempotent_on_fresh_db():
     """Re-running the migration on the same DB must be a no-op (PRAGMA
-    user_version already at the latest version — currently 10 after
+    user_version already at the latest version — currently 11 after
     0004-0006 line_reconciliation, 0005 ta1_acks, SP9's 0007
     providers/payers/clearhouse, SP10's 0008 payer_rejected,
-    SP11's 0009 audit_log, and SP14's 0010 payer_rejected_acknowledged)."""
+    SP11's 0009 audit_log, SP14's 0010 payer_rejected_acknowledged,
+    SP16's 0011 processed_inbound_files)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 10
+    assert v1 == 11
     # A second run should not raise and should not bump the version.
     db_migrate.run(db.engine())
     with db.engine().begin() as c:
         v2 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v2 == 10
+    assert v2 == 11
 
 
 def test_add_ack_persists_row():