feat: replace PM2 with systemd --user services for production

Runs tssbot-web, tssbot-webhook, and tssbot-backend as systemd --user
units instead of PM2 processes. tssbot-web moves from a 2-worker PM2
cluster to a single instance, so deploys now restart it directly
instead of doing a zero-downtime cluster reload.

webhook.cjs now shells out to `systemctl --user restart` instead of
`pm2 reload`, and PM2_RESTART_TARGETS/WEBHOOK_PM2_NAME are renamed to
RESTART_TARGETS/WEBHOOK_SERVICE_NAME. scripts/install-systemd-services.sh
symlinks the new unit files into ~/.config/systemd/user and enables them.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

systemd/tssbot-backend.service

@@ -0,0 +1,19 @@
+[Unit]
+Description=tssbot backend API service
+After=network-online.target
+Wants=network-online.target
+StartLimitIntervalSec=100
+StartLimitBurst=10
+
+[Service]
+Type=simple
+WorkingDirectory=%h/tssbot.web
+ExecStart=%h/tssbot.web/backend/target/release/tssbot-backend
+Restart=on-failure
+RestartSec=200ms
+RestartSteps=10
+RestartMaxDelaySec=10s
+TimeoutStopSec=10
+
+[Install]
+WantedBy=default.target