v0.13.2 — status-alert context, persistent arming, interval knob

Operational polish around the live detection:

• Configurable analysis cadence: DETECT_INTERVAL / --detect-interval
  (seconds between analyzed frames; the analysis adds no network traffic —
  the camera stream is the only real load, and it now runs only while a
  UI client is open or a print is being monitored).
• Status alerts with context: pause/abort/finish Telegram messages
  append the job name and layer/progress; on a CC2 an empty filament
  sensor during a pause is reported as "(filament runout)"; raw firmware
  status codes land in the journal for future mapping.
• Persistent arming: the armed response (notify/pause/stop) survives
  service restarts; DETECT_ACTION is the initial default.
• Camera-state documentation: README explains exactly when the camera is
  on; detector alerts sharpened; docs got a plain-language pass.