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>

composer.json

@@ -20,6 +20,7 @@
         "phpoffice/phpspreadsheet": "^5.5",
         "phpstan/phpdoc-parser": "^2.3",
         "sabre/vobject": "^4.5",
+        "sentry/sentry-symfony": "^5.10",
         "symfony/asset": "8.0.*",
         "symfony/console": "8.0.*",
         "symfony/doctrine-messenger": "^8.0",