feat: add Bluetooth adapter power state handling and auto-power-on

Addresses the common "Not Powered" error where Bluetooth adapters are
powered off, preventing BLE operations. This issue is particularly common
on Raspberry Pi after boot or system updates.

Changes:

1. README.md - Added comprehensive troubleshooting section
   - New section: "Bluetooth adapter not powered"
   - Documented symptoms, causes, and 4 solution methods
   - Instructions for automatic power-on at boot
   - Cross-referenced from other sections

2. BLEInterface.py - Enhanced error handling in _discover_peers()
   - Detect "Not Powered" errors specifically
   - Show clear, actionable error messages instead of stack traces
   - Provide direct solution commands
   - Link to troubleshooting documentation
   - Gracefully handle error without crashing

3. install.sh - Automatic power state checking
   - New "Step 5B: Bluetooth Adapter Power State" section
   - Check if adapter is powered using `bluetoothctl show`
   - Automatically power on adapter if needed
   - Verify operation succeeded
   - Provide troubleshooting steps if power-on fails
   - Check for rfkill blocks

4. examples/bluetooth-power-on.service - New systemd service
   - Ensures Bluetooth is powered on at boot
   - Optional but recommended for production
   - Includes installation instructions in README

5. examples/config_example.toml - Added troubleshooting entry #7
   - Documents power state issue in config comments
   - Cross-references systemd service example
   - Notes that installer (v1.x+) handles this automatically

Impact:
- Users get clear guidance instead of cryptic stack traces
- Installer automatically fixes the issue during setup
- Reduces support burden for common power state errors
- Enables automatic recovery at boot via systemd service

Fixes: "Not Powered" / "No powered Bluetooth adapters found" errors
Tested on: Raspberry Pi Zero 2 W with Raspberry Pi OS Lite 64-bit

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -250,6 +250,50 @@ These errors occur when BlueZ attempts Classic Bluetooth (BR/EDR) connections in
 **Solution:**
 Enable BlueZ experimental mode (see Installation → Manual Installation → step 4). If you used the automated installer, re-run it without `--skip-experimental`.
 
+### Bluetooth adapter not powered / "No powered Bluetooth adapters found"
+The Bluetooth adapter exists but is powered off, preventing BLE operations.
+
+**Symptoms:**
+- Error: `dbus.exceptions.DBusException: org.bluez.Error.Failed: Not Powered`
+- Error: `BleakError: No powered Bluetooth adapters found.`
+- BLE interface fails to start or discover peers
+- GATT server startup fails immediately
+
+**Cause:**
+The Bluetooth adapter is in a powered-off state. This commonly happens on Raspberry Pi after boot or system updates.
+
+**Solution:**
+Power on the Bluetooth adapter:
+
+```bash
+# Option 1: Using bluetoothctl (recommended)
+bluetoothctl power on
+
+# Option 2: If adapter is RF-blocked
+sudo rfkill unblock bluetooth
+
+# Option 3: Using hciconfig (older systems)
+sudo hciconfig hci0 up
+
+# Verify adapter is powered:
+bluetoothctl show
+# Should display "Powered: yes"
+```
+
+**Automatic power-on at boot:**
+Ensure Bluetooth service is enabled and starts at boot:
+
+```bash
+# Enable Bluetooth service
+sudo systemctl enable bluetooth
+sudo systemctl start bluetooth
+
+# For persistent power-on, create a systemd service:
+# See examples/bluetooth-power-on.service
+```
+
+The automated installer (v1.x+) automatically checks and powers on the Bluetooth adapter during installation.
+
 ## Architecture
 
 The BLE interface consists of four main components: