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>

symfony.lock

@@ -124,6 +124,15 @@
             "bin/phpunit"
         ]
     },
+    "sentry/sentry-symfony": {
+        "version": "5.10",
+        "recipe": {
+            "repo": "github.com/symfony/recipes-contrib",
+            "branch": "main",
+            "version": "5.0",
+            "ref": "aac2bc5220e9ab5b9e3838a7a4da90e7f74e6148"
+        }
+    },
     "symfony/console": {
         "version": "8.0",
         "recipe": {