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/postinst

@@ -14,8 +14,13 @@ case "$1" in
         chown sensorpajen:sensorpajen /etc/sensorpajen
         chmod 750 /etc/sensorpajen
 
+        # Create state directory with proper permissions (writable at runtime)
+        mkdir -p /var/lib/sensorpajen
+        chown sensorpajen:sensorpajen /var/lib/sensorpajen
+        chmod 750 /var/lib/sensorpajen
+
         # Copy example configs to /etc/sensorpajen if they don't exist
-        for sample in sensorpajen.env.example sensors.json.example discovered_sensors.json.example; do
+        for sample in sensorpajen.env.example sensors.json.example; do
             source_file="/usr/share/doc/sensorpajen/examples/$sample"
             target_file="/etc/sensorpajen/${sample%.example}"