feat: debian packaging, release pipeline, and build scripts

- debian/postinst: create daena user, directories, reload systemd
- debian/prerm: stop and disable service on removal
- scripts/build-deb.sh: build self-contained .deb with all deps
- CI: add release job triggered by v* tags, builds wheel + .deb,
  uploads both to GitHub Release

debian/prerm

@@ -0,0 +1,11 @@
+#!/bin/sh
+set -e
+
+case "$1" in
+    remove|upgrade|deconfigure)
+        if command -v systemctl >/dev/null 2>&1; then
+            systemctl stop daena.service >/dev/null 2>&1 || true
+            systemctl disable daena.service >/dev/null 2>&1 || true
+        fi
+        ;;
+esac