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/control

@@ -0,0 +1,15 @@
+Package: irc-bot
+Version: 1.0.0
+Section: net
+Priority: optional
+Architecture: all
+Installed-Size: 0
+Depends: python3 (>= 3.11), python3-pip, python3-venv, adduser
+Recommends: sqlite3
+Maintainer: tocmo0nlord <tocmo0nlord@activeblue.net>
+Description: Active Blue IRC LLM Bot
+ An IRC bot that connects to a ZNC bouncer, joins configured channels,
+ and responds to users via a locally hosted Ollama LLM. Includes a
+ Flask-based web admin portal for live configuration. Conversation
+ history is persisted per user per channel in SQLite.
+Homepage: http://192.168.1.64:3000/tocmo0nlord/irc-bot