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>

bot/memory.py

@@ -5,7 +5,11 @@ import re
 
 logger = logging.getLogger(__name__)
 
-HISTORY_DIR = "data/history"
+HISTORY_DIR = (
+    "/var/lib/irc-bot/history"
+    if os.path.isdir("/var/lib/irc-bot")
+    else "data/history"
+)
 
 
 def _sanitize_channel(channel: str) -> str: