Add Debian .deb package support

- debian/control, changelog, conffiles — package metadata
- debian/postinst — creates irc-bot system user, installs Python venv,
  symlinks runtime dirs, enables systemd services
- debian/prerm/postrm — clean stop/uninstall with purge support
- debian/systemd/ — hardened systemd units for bot and portal
- build_deb.sh — assembles and builds the .deb via dpkg-deb
- Path resolver in irc_client.py, memory.py, config_manager.py, portal/app.py:
  uses /etc/irc-bot + /var/lib/irc-bot when installed as .deb, relative
  paths otherwise (Docker/venv unchanged)

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

debian/postrm

@@ -0,0 +1,25 @@
+#!/bin/bash
+set -e
+
+case "$1" in
+  purge)
+    # Remove data, logs, config only on purge (not plain remove)
+    rm -rf /var/lib/irc-bot /var/log/irc-bot /etc/irc-bot
+    # Remove system user
+    if getent passwd irc-bot > /dev/null 2>&1; then
+      deluser --system irc-bot 2>/dev/null || true
+    fi
+    if getent group irc-bot > /dev/null 2>&1; then
+      delgroup --system irc-bot 2>/dev/null || true
+    fi
+    if [ -d /run/systemd/system ]; then
+      systemctl daemon-reload
+    fi
+    ;;
+  remove)
+    # Remove the venv on plain remove (can be rebuilt on reinstall)
+    rm -rf /opt/irc-bot/venv
+    ;;
+esac
+
+exit 0