fix: Correct systemd configuration and Debian package setup

- Fix database update service to include config path parameter
- Move ConditionACPower from [Timer] to [Unit] section in timer config
- Add database directory to ReadWritePaths in main service
- Set explicit database path in Debian package config template
- Simplify postinst script to be quiet and use consistent skyview-adsb user
- Update README.Debian with comprehensive systemd service documentation
- Remove confusing dual-user setup in favor of single skyview-adsb user

SystemD Configuration Summary:
- Main service: skyview-adsb.service with database pre-update
- Database service: skyview-database-update.service (oneshot)
- Weekly timer: skyview-database-update.timer (Sunday 3AM with randomization)
- All services use skyview-adsb user consistently
- Database path: /var/lib/skyview-adsb/skyview.db
- Config path: /etc/skyview-adsb/config.json

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

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

debian/lib/systemd/system/skyview-database-update.timer

@@ -2,6 +2,8 @@
 Description=SkyView Database Update Timer
 Documentation=man:skyview-data(1)
 Requires=skyview-database-update.service
+# Don't start if system is on battery (laptops)
+ConditionACPower=true
 
 [Timer]
 # Run weekly on Sunday at 3 AM
@@ -10,8 +12,6 @@ OnCalendar=Sun 03:00
 RandomizedDelaySec=3600
 # Start immediately if system was down during scheduled time
 Persistent=true
-# Don't start if system is on battery (laptops)
-ConditionACPower=true
 
 [Install]
 WantedBy=timers.target