Add Debian package and Gitea APT repository support

debian/control, postinst, prerm, postrm — standard dpkg package lifecycle
debian/files/opt/dupfinder/dupfinder-setup.sh — interactive setup:
  checks Docker, detects NVIDIA GPU, prompts for photos/data paths,
  writes docker-compose.override.yml with GPU reservation if present,
  pulls image from registry (builds from source as fallback)
debian/files/usr/local/bin/dupfinder — CLI wrapper:
  setup / start / stop / restart / status / logs / open / update
debian/files/etc/systemd/system/dupfinder.service — systemd unit,
  guards against starting before setup has run
debian/build-deb.sh — builds .deb and uploads to Gitea package registry;
  prints the exact apt sources.list line on success

Install on any Debian/Ubuntu machine:
  echo "deb [trusted=yes] http://192.168.1.64:3000/api/packages/tocmo0nlord/debian bookworm main" \
    | sudo tee /etc/apt/sources.list.d/dupfinder.list
  sudo apt update && sudo apt install dupfinder
  sudo dupfinder setup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

debian/files/etc/systemd/system/dupfinder.service

@@ -0,0 +1,33 @@
+[Unit]
+Description=DupFinder Duplicate Photo Scanner
+Documentation=http://192.168.1.64:3000/tocmo0nlord/duplicate-finder
+After=docker.service network-online.target
+Requires=docker.service
+Wants=network-online.target
+
+[Service]
+Type=simple
+Restart=on-failure
+RestartSec=10
+EnvironmentFile=-/etc/dupfinder.conf
+WorkingDirectory=/opt/dupfinder
+
+ExecStart=/usr/bin/docker compose \
+    -f /opt/dupfinder/docker-compose.yml \
+    -f /opt/dupfinder/docker-compose.override.yml \
+    up --no-build --remove-orphans
+
+ExecStop=/usr/bin/docker compose \
+    -f /opt/dupfinder/docker-compose.yml \
+    -f /opt/dupfinder/docker-compose.override.yml \
+    down
+
+# Don't start if override hasn't been created yet (setup not run)
+ExecStartPre=/bin/test -f /opt/dupfinder/docker-compose.override.yml
+
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=dupfinder
+
+[Install]
+WantedBy=multi-user.target