feat(observability) : error tracking GlitchTip back + front (INFRA #146)

Backend : sentry/sentry-symfony branché en prod uniquement (bundle prod-only,
exceptions seules, 4xx ignorés, release = app.version), DSN via SENTRY_DSN
(runtime, infra/prod/.env).
Frontend : @sentry/nuxt chargé seulement si NUXT_PUBLIC_SENTRY_DSN présent
(donc au build prod), upload des source maps gated sur les secrets. DSN front
et secrets passés en build-args (Dockerfile) depuis les secrets Gitea (CI).
Doc README (section Error tracking) + .env.example.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

infra/prod/deploy.sh

@@ -37,6 +37,14 @@ echo "==> Clearing cache..."
 sudo docker compose exec -T -u www-data app php bin/console cache:clear --env=prod
 sudo docker compose exec -T -u www-data app php bin/console cache:warmup --env=prod
 
+echo "==> Checking error tracking (GlitchTip)..."
+SENTRY_DSN_VALUE="$(sudo docker compose exec -T app printenv SENTRY_DSN 2>/dev/null || true)"
+if [ -n "${SENTRY_DSN_VALUE}" ]; then
+  echo "    SENTRY_DSN detecte — erreurs backend envoyees a GlitchTip."
+else
+  echo "    SENTRY_DSN absent/vide — error tracking backend desactive (ajouter SENTRY_DSN dans .env)."
+fi
+
 echo "==> Disabling maintenance mode..."
 rm -f maintenance.on