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/postinst

@@ -0,0 +1,64 @@
+#!/bin/bash
+set -e
+
+APP_DIR=/opt/irc-bot
+DATA_DIR=/var/lib/irc-bot
+LOG_DIR=/var/log/irc-bot
+CONF_DIR=/etc/irc-bot
+USER=irc-bot
+GROUP=irc-bot
+
+case "$1" in
+  configure)
+    # Create system user/group if they don't exist
+    if ! getent group "$GROUP" > /dev/null 2>&1; then
+      addgroup --system "$GROUP"
+    fi
+    if ! getent passwd "$USER" > /dev/null 2>&1; then
+      adduser --system --ingroup "$GROUP" --no-create-home \
+              --home "$DATA_DIR" --shell /usr/sbin/nologin "$USER"
+    fi
+
+    # Runtime directories
+    install -d -m 750 -o "$USER" -g "$GROUP" "$DATA_DIR"
+    install -d -m 750 -o "$USER" -g "$GROUP" "$DATA_DIR/history"
+    install -d -m 750 -o "$USER" -g "$GROUP" "$LOG_DIR"
+
+    # Config directory — preserve existing .env if present
+    install -d -m 750 -o "$USER" -g "$GROUP" "$CONF_DIR"
+    if [ ! -f "$CONF_DIR/.env" ]; then
+      cp "$APP_DIR/.env.example" "$CONF_DIR/.env"
+      chown "$USER:$GROUP" "$CONF_DIR/.env"
+      chmod 640 "$CONF_DIR/.env"
+      echo ""
+      echo "  ┌─────────────────────────────────────────────────────┐"
+      echo "  │  irc-bot: edit /etc/irc-bot/.env before starting   │"
+      echo "  │  Set ZNC_USER, ZNC_PASSWORD, ZNC_NETWORK at minimum │"
+      echo "  └─────────────────────────────────────────────────────┘"
+      echo ""
+    fi
+
+    # Symlink runtime directories into app dir so relative paths work
+    ln -sfn "$DATA_DIR"   "$APP_DIR/data"
+    ln -sfn "$LOG_DIR"    "$APP_DIR/logs"
+    ln -sfn "$CONF_DIR"   "$APP_DIR/config"
+
+    # Install Python venv with dependencies
+    if [ ! -d "$APP_DIR/venv" ]; then
+      echo "irc-bot: creating Python venv and installing dependencies..."
+      python3 -m venv "$APP_DIR/venv"
+      "$APP_DIR/venv/bin/pip" install --quiet --no-cache-dir -r "$APP_DIR/requirements.txt"
+      chown -R "$USER:$GROUP" "$APP_DIR/venv"
+    fi
+
+    # Enable and start systemd services
+    if [ -d /run/systemd/system ]; then
+      systemctl daemon-reload
+      systemctl enable irc-bot.service irc-bot-portal.service
+      echo "irc-bot: services enabled. Start with:"
+      echo "  systemctl start irc-bot irc-bot-portal"
+    fi
+    ;;
+esac
+
+exit 0