feat: add debian packaging, CI/CD and update script

- Add debian/ packaging for .deb generation
- Add .forgejo/workflows/build.yml for CI/CD (runner label: docker)
- Add scripts/fullupdater and scripts/fullupdater-update
- Remove obsolete install.sh
- Update .gitignore for debian build artifacts

debian/postinst

@@ -0,0 +1,23 @@
+#!/bin/bash
+set -e
+
+VENV_DIR="/opt/full-updater/.venv"
+OPT_DIR="/opt/full-updater"
+
+# Create venv and install deps
+if [ ! -d "$VENV_DIR" ]; then
+    python3 -m venv "$VENV_DIR"
+fi
+"$VENV_DIR/bin/pip" install --upgrade pip
+"$VENV_DIR/bin/pip" install -r "$OPT_DIR/requirements.txt"
+
+# Ensure symlinks
+ln -sf "$OPT_DIR/scripts/fullupdater" /usr/local/bin/fullupdater
+
+# Ensure cache dir exists
+mkdir -p /tmp/full-updater-cache
+
+# Upgrade existing install if deps changed
+"$VENV_DIR/bin/pip" install -r "$OPT_DIR/requirements.txt" --upgrade
+
+exit 0