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>

examples/config_example.toml

@@ -266,6 +266,15 @@ power_mode = balanced
 #    - If you used install.sh, it should have enabled this automatically
 #    - Verify: ps aux | grep bluetoothd (should show -E flag)
 #
+# 7. Bluetooth adapter not powered (No powered Bluetooth adapters found):
+#    IMPORTANT: BLE interface won't work if adapter is powered off!
+#    - Symptoms: "Not Powered" errors, interface fails to start, no peer discovery
+#    - Solution: bluetoothctl power on
+#    - Verify: bluetoothctl show (should display "Powered: yes")
+#    - For automatic power-on at boot: see examples/bluetooth-power-on.service
+#    - If blocked: sudo rfkill unblock bluetooth
+#    - The installer (v1.x+) automatically powers on the adapter
+#
 # For more troubleshooting, see README.md
 
 # ============================================================================