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

- Lower DEFAULT_DETECTION_THRESHOLD from 0.9 to 0.6 (IR cameras score
  lower due to grayscale/low-contrast images; 0.9 prevents any detection)
- Increase IR_TRIGGER_DELAY_MS from 200ms to 1500ms (Lenovo emitters
  need more time to activate before frame capture)
- Increase CAMERA_WARMUP_FRAMES (10->15) and CAMERA_WARMUP_DELAY_MS
  (100->200ms) for IR exposure stabilization
- Always save debug frame on enroll failure with brightness and score info
- Add YuNet face score logging to help diagnose threshold issues
- Add docs/LENOVO_IR_DEBUG.md with full troubleshooting guide

Tested on Lenovo with Ubuntu 25.04, /dev/video2 IR camera.

Author: amletoflorio

LENOVO_IR_DEBUG.md

@@ -0,0 +1,163 @@
+# Lenovo IR Camera — Debug Guide
+
+## Symptom
+
+```
+Response: ENROLL_FAIL Found 0 faces in ir. Expecting exactly 1.
+```
+
+## Root Cause
+
+The YuNet face detector assigns **significantly lower confidence scores** to faces
+captured by IR cameras (grayscale image, low contrast, often overexposed).
+
+The default threshold of `0.9` was tuned for standard RGB webcams. On IR cameras
+it is nearly impossible to reach. Lowering it to `0.5–0.6` resolves the issue
+in most cases.
+
+---
+
+## Quick Fix (no recompilation required)
+
+Edit `/etc/linuxcampam/config.ini`:
+
+```ini
+[Auth]
+detection_threshold = 0.5
+
+[Capture]
+enroll_averaging = on
+enroll_average_frames = 7
+```
+
+Then restart the daemon:
+```bash
+sudo systemctl restart linuxcampam
+```
+
+---
+
+## Diagnosing with logs
+
+During enrollment, watch the logs in real time:
+
+```bash
+journalctl -u linuxcampam -f
+```
+
+What to look for:
+
+| Log message | Meaning |
+|-------------|---------|
+| `0 faces found above threshold (0.9)` | Threshold too high → lower it |
+| `Best score: 0.65 \| Threshold: 0.9` | Face found but below threshold → lower to 0.55 |
+| `Brightness: 12` | Frame nearly black → IR emitter not activating |
+| `Brightness: 240` | Frame overexposed → camera exposure issue |
+
+---
+
+## Inspecting the failed frame
+
+On every failed enrollment the captured frame is automatically saved to:
+
+```
+/var/log/linuxcampam/failed_enroll_ir_<username>.jpg
+```
+
+Open it to check what the camera actually captured:
+```bash
+xdg-open /var/log/linuxcampam/failed_enroll_ir_<username>.jpg
+```
+
+- **Nearly black frame** → IR emitter did not activate (see section below)
+- **Grainy / blurry frame** → increase `enroll_average_frames`
+- **Face visible but not detected** → lower `detection_threshold` further
+- **No face in frame** → positioning issue during enrollment
+
+---
+
+## IR emitter not activating
+
+If the saved frame is nearly black:
+
+1. Verify `linux-enable-ir-emitter` is installed:
+   ```bash
+   ls /usr/local/bin/linux-enable-ir-emitter
+   ```
+
+2. Test it manually:
+   ```bash
+   sudo linux-enable-ir-emitter run
+   ```
+
+3. If not installed:
+   ```bash
+   sudo apt install linux-enable-ir-emitter
+   # or from source: https://github.com/EmixamPP/linux-enable-ir-emitter
+   ```
+
+4. Configure for your specific hardware model:
+   ```bash
+   sudo linux-enable-ir-emitter configure
+   ```
+   Follow the interactive procedure — move your head in front of the camera
+   while it tries different configurations. When the emitter blinks and you see
+   `The infrared emitter has been successfully enabled!` you are done.
+
+---
+
+## Finding the correct IR camera device
+
+```bash
+# List all webcams
+v4l2-ctl --list-devices
+
+# Identify the IR camera by checking shape and brightness
+python3 -c "
+import cv2
+for i in range(4):
+    cap = cv2.VideoCapture(i)
+    if not cap.isOpened():
+        print(f'video{i}: could not open')
+        continue
+    ret, frame = cap.read()
+    cap.release()
+    if ret:
+        print(f'video{i}: shape={frame.shape} brightness={frame.mean():.0f}')
+"
+```
+
+The IR camera has **1 channel** (shape like `(360, 640)`) instead of 3 (RGB).
+On Lenovo laptops it is typically `/dev/video2`.
+
+---
+
+## Recommended settings for Lenovo ThinkPad / IdeaPad
+
+```ini
+[Auth]
+detection_threshold = 0.5
+timeout_ms = 5000
+
+[Capture]
+enroll_hdr = off               ; IR cameras do not support HDR
+enroll_averaging = on
+enroll_average_frames = 7
+
+[Hardware]
+camera_path_ir = /dev/video2   ; verify with v4l2-ctl --list-devices
+```
+
+---
+
+## Code changes (this fix)
+
+| File | Change |
+|------|--------|
+| `include/constants.hpp` | `IR_TRIGGER_DELAY_MS`: 200 → 1500 ms |
+| `include/constants.hpp` | `CAMERA_WARMUP_FRAMES`: 10 → 15 |
+| `include/constants.hpp` | `CAMERA_WARMUP_DELAY_MS`: 100 → 200 ms |
+| `src/service/config.hpp` | `DEFAULT_DETECTION_THRESHOLD`: 0.9 → 0.6 |
+| `src/service/auth_engine.cpp` | Log YuNet score + brightness in `generateEmbedding` |
+| `src/service/auth_engine.cpp` | Always save failed enrollment frame with diagnostic info |
+| `config/config.ini` | Updated default `detection_threshold` + averaging enabled |