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>

frontend/package.json

@@ -16,6 +16,7 @@
         "@nuxtjs/i18n": "^10.2.3",
         "@nuxtjs/tailwindcss": "^6.14.0",
         "@pinia/nuxt": "^0.11.3",
+        "@sentry/nuxt": "^10.61.0",
         "@tailwindcss/typography": "^0.5.19",
         "@vuepic/vue-datepicker": "^12.1.0",
         "chart.js": "^4.5.1",