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/postinst

@@ -0,0 +1,27 @@
+#!/bin/bash
+set -e
+
+# Create data directory with correct permissions
+mkdir -p /var/lib/dupfinder/data
+chmod 755 /var/lib/dupfinder
+
+# Reload systemd and enable service (don't start yet — needs user config first)
+systemctl daemon-reload
+systemctl enable dupfinder.service 2>/dev/null || true
+
+echo ""
+echo "╔══════════════════════════════════════════════════╗"
+echo "║         DupFinder installed successfully!        ║"
+echo "╠══════════════════════════════════════════════════╣"
+echo "║                                                  ║"
+echo "║  Run setup to configure your photos path:        ║"
+echo "║                                                  ║"
+echo "║    sudo dupfinder setup                          ║"
+echo "║                                                  ║"
+echo "║  After setup, manage with:                       ║"
+echo "║    sudo systemctl start dupfinder                ║"
+echo "║    sudo systemctl stop  dupfinder                ║"
+echo "║    dupfinder status                              ║"
+echo "║                                                  ║"
+echo "╚══════════════════════════════════════════════════╝"
+echo ""