Fix Bluetooth permission error with AmbientCapabilities and enhanced postinst

- Add AmbientCapabilities=CAP_NET_RAW CAP_NET_ADMIN to systemd service files
- Add sensorpajen user to bluetooth group in postinst
- Improve setcap error handling in postinst with clearer messaging
- Add comprehensive troubleshooting section for Bluetooth permission errors

This fixes the 'Operation not permitted' error when the service tries to
access Bluetooth hardware. The fix uses two layers of protection:
1. systemd AmbientCapabilities (modern, robust)
2. File capabilities via setcap (traditional, wider compatibility)

debian/README.md

@@ -287,16 +287,51 @@ Common issues:
 - sensors.json is empty
 - Bluetooth adapter not available
 
-### Bluetooth Capability Not Set
+### Bluetooth Permission Error: "Operation not permitted"
+
+If the service fails with `PermissionError: [Errno 1] Operation not permitted`:
 
 ```bash
-# Manually set capability
+# Check current capabilities
+getcap $(readlink -f /opt/sensorpajen/venv/bin/python3)
+# Should show: cap_net_admin,cap_net_raw+eip
+```
+
+**Solution 1: Re-apply file capabilities** (Quick fix)
+
+```bash
+# Set capabilities on Python executable
 sudo setcap cap_net_raw,cap_net_admin+eip $(readlink -f /opt/sensorpajen/venv/bin/python3)
 
 # Verify
 getcap $(readlink -f /opt/sensorpajen/venv/bin/python3)
+
+# Restart service
+sudo systemctl restart sensorpajen
 ```
 
+**Solution 2: Add user to bluetooth group**
+
+```bash
+sudo usermod -aG bluetooth sensorpajen
+sudo systemctl restart sensorpajen
+```
+
+**Solution 3: Verify systemd capabilities**
+
+The service file uses `AmbientCapabilities=CAP_NET_RAW CAP_NET_ADMIN` as a fallback. Verify it's configured:
+
+```bash
+sudo systemctl cat sensorpajen | grep -A2 Capabilities
+```
+
+**Why this happens**: File capabilities can be lost when:
+- Python is upgraded/reinstalled
+- Filesystem is mounted with `nosuid`
+- The venv is recreated
+
+The systemd service now uses both `AmbientCapabilities` (modern approach) and file capabilities (setcap) for maximum compatibility.
+
 ## Development Workflow
 
 ### Making Changes