fix: preserve systemd enable/disable state on package upgrade

The preremove script was unconditionally stopping and disabling the
service, which meant upgrades (dpkg -i new.deb) would disable the
service. Users had to manually re-enable after every upgrade.

Now:
- preremove: only stop+disable on actual removal (not upgrade)
  Checks $1 for "remove"/"purge" (deb) or "0" (rpm)
- postinstall: restart the service on upgrade if it was running,
  preserving enable/disable state. Only shows first-install
  instructions on initial install.

Tested with shellcheck.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dist/postinstall.sh

@@ -1,6 +1,9 @@
 #!/bin/sh
 # Post-install script for Favoritter .deb/.rpm package.
-# Creates the system user and sets directory permissions.
+# Creates the system user, sets directory permissions, and handles upgrades.
+#
+# Debian/Ubuntu: called with "configure" on install/upgrade.
+# RPM: called with 1 on first install, 2+ on upgrade.
 set -e
 
 # Create system user if it doesn't exist.
@@ -12,10 +15,25 @@ fi
 install -d -o favoritter -g favoritter -m 0750 /var/lib/favoritter
 install -d -o favoritter -g favoritter -m 0750 /var/lib/favoritter/uploads
 
-# Reload systemd to pick up the service file.
+# Reload systemd to pick up any service file changes.
 if command -v systemctl >/dev/null 2>&1; then
     systemctl daemon-reload
+
+    # On upgrade: restart the service if it was running.
+    # This picks up the new binary without losing enable/disable state.
+    if systemctl is-active --quiet favoritter 2>/dev/null; then
+        systemctl restart favoritter
+    fi
 fi
 
+# Only show the setup message on first install (not upgrades).
+action="${1:-}"
+case "$action" in
+    # Debian first install or RPM first install ($1=1).
+    configure|1)
+        if ! systemctl is-enabled --quiet favoritter 2>/dev/null; then
             echo "Favoritter installed. Configure /etc/favoritter/favoritter.env then run:"
             echo "  sudo systemctl enable --now favoritter"
+        fi
+        ;;
+esac

dist/preremove.sh

@@ -1,9 +1,30 @@
 #!/bin/sh
 # Pre-remove script for Favoritter .deb/.rpm package.
-# Stops the service before package removal.
+# Only stops and disables the service on actual removal, not on upgrade.
+#
+# Debian/Ubuntu: called with "remove" on uninstall, "upgrade" on upgrade.
+# RPM (Fedora/RHEL): called with 0 on final removal, 1+ on upgrade.
 set -e
 
+action="${1:-}"
+
+case "$action" in
+    # Debian: full removal.
+    remove|purge)
         if command -v systemctl >/dev/null 2>&1; then
             systemctl stop favoritter 2>/dev/null || true
             systemctl disable favoritter 2>/dev/null || true
         fi
+        ;;
+    # RPM: $1 is the number of remaining installations.
+    0)
+        if command -v systemctl >/dev/null 2>&1; then
+            systemctl stop favoritter 2>/dev/null || true
+            systemctl disable favoritter 2>/dev/null || true
+        fi
+        ;;
+    # Debian "upgrade" or RPM "1+" — do nothing, the service stays running.
+    # The new postinstall will daemon-reload and restart.
+    *)
+        ;;
+esac