feat : error tracking backend vers GlitchTip (prod-only, inerte sans DSN)

Ajout du SDK sentry/sentry-symfony enregistré prod-only, config sentry.yaml
(DSN runtime, release app.version, 4xx ignorés, pas de tracing) et handler
Monolog ERROR+. Sans SENTRY_DSN le SDK est no-op. Transport réseau via
Tailscale (infra, hors repo).

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

composer.json

@@ -17,6 +17,7 @@
         "nelmio/cors-bundle": "^2.6",
         "phpdocumentor/reflection-docblock": "^5.6",
         "phpstan/phpdoc-parser": "^2.3",
+        "sentry/sentry-symfony": "^5.10",
         "symfony/asset": "8.0.*",
         "symfony/console": "8.0.*",
         "symfony/dotenv": "8.0.*",