Rewrite Pastel from Python to Go with watchlist feature

Full rewrite of the gaming deals Matrix bot in Go for better performance
and a single static binary. Includes all original functionality (CheapShark,
ITAD, Epic free games, multi-currency pricing, dedup, preflight checks,
presence heartbeat) plus a new watchlist feature where users can DM the bot
to watch for specific game deals with 180-day auto-expiry.

New: systemd service file, DB migration script for Python-to-Go transition.

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

pastel.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=Pastel — Matrix gaming deals bot
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+WorkingDirectory=/opt/pastel
+ExecStart=/opt/pastel/pastel
+EnvironmentFile=/opt/pastel/.env
+Restart=on-failure
+RestartSec=10
+
+# Hardening
+NoNewPrivileges=true
+ProtectSystem=strict
+ProtectHome=true
+ReadWritePaths=/opt/pastel
+PrivateTmp=true
+
+[Install]
+WantedBy=multi-user.target