Fix system installation state directory

The service was failing with 'Read-only file system' when trying to create
discovered_sensors.json in the /etc/sensorpajen config directory.

Changes:
- config.py: Add STATE_DIR for runtime state
  - System mode: /var/lib/sensorpajen (writable at runtime)
  - Dev mode: config/ (same as config directory)
- config.py: Use STATE_DIR for discovered_sensors.json path
- debian/postinst: Create and own /var/lib/sensorpajen
- debian/sensorpajen.service: Add /var/lib/sensorpajen to ReadWritePaths
- debian/postinst: Remove discovered_sensors.json.example copy (created at runtime)

This separates:
- Config: /etc/sensorpajen (static, not updated by service)
- State: /var/lib/sensorpajen (dynamic, updated by service at runtime)

debian/sensorpajen.service

@@ -26,7 +26,7 @@ SyslogIdentifier=sensorpajen
 PrivateTmp=true
 ProtectSystem=strict
 ProtectHome=true
-ReadWritePaths=/etc/sensorpajen
+ReadWritePaths=/etc/sensorpajen /var/lib/sensorpajen
 
 [Install]
 WantedBy=multi-user.target