fix: lower detection threshold and improve IR camera timing for Lenovo

[amletoflorio]

Problem

On Lenovo laptops with IR cameras, enrollment fails with:
ENROLL_FAIL Found 0 faces in ir. Expecting exactly 1.

Root causes

1. detection_threshold = 0.9 is too high for IR cameras — YuNet scores
   rarely exceed 0.7 on grayscale/low-contrast IR images.
2. IR_TRIGGER_DELAY_MS = 200ms is too short for Lenovo IR emitters,
   resulting in near-black frames (brightness ~14/255) at capture time.
3. Insufficient warmup frames for IR exposure stabilization.

Changes

• Lower DEFAULT_DETECTION_THRESHOLD from 0.9 to 0.6
• Increase IR_TRIGGER_DELAY_MS from 200ms to 1500ms
• Increase CAMERA_WARMUP_FRAMES (10→15) and CAMERA_WARMUP_DELAY_MS (100→200ms)
• Always save failed enrollment frame with brightness + YuNet score in logs
• Add docs/LENOVO_IR_DEBUG.md with full troubleshooting guide

Tested on

Lenovo laptop, Ubuntu 26.04, kernel 7.x, IR camera on /dev/video2

[Vladush]

@amletoflorio Thanks for opening the PR. I'll take a look on it as soon as my schedule will allow me that.
In the mean time - please note, that the whole development and main testing was done on my Lenovo Yoga Pro 7 (14ASP9) - so all defaults are working perfectly fine here. Also checked on Lenovo Thinkpad and worked fine too.
So, changing defaults, which can be adjusted in config and may be different on each system - may not be the best idea.