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/fixtures/minimal_ta1.txt

@@ -0,0 +1 @@
+ISA*00*          *00*          *ZZ*SENDER         *ZZ*RECEIVER       *260520*1750*^*00501*000000001*0*P*:~TA1*000000001*20260520*1750*A*000*20260520~IEA*1*000000001~