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>

config/bundles.php

@@ -8,6 +8,7 @@ use Doctrine\Bundle\FixturesBundle\DoctrineFixturesBundle;
 use Doctrine\Bundle\MigrationsBundle\DoctrineMigrationsBundle;
 use Lexik\Bundle\JWTAuthenticationBundle\LexikJWTAuthenticationBundle;
 use Nelmio\CorsBundle\NelmioCorsBundle;
+use Sentry\SentryBundle\SentryBundle;
 use Symfony\AI\McpBundle\McpBundle;
 use Symfony\Bundle\FrameworkBundle\FrameworkBundle;
 use Symfony\Bundle\MonologBundle\MonologBundle;
@@ -24,4 +25,5 @@ return [
     LexikJWTAuthenticationBundle::class => ['all' => true],
     McpBundle::class                    => ['all' => true],
     MonologBundle::class                => ['all' => true],
+    SentryBundle::class                 => ['prod' => true],
 ];