fix(deps) : add symfony/mime as explicit dependency

Was previously pulled as transitive dependency but disappeared after
composer update, causing 500 errors on document upload in production.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

composer.json

@@ -23,6 +23,7 @@
         "symfony/flex": "^2",
         "symfony/framework-bundle": "8.0.*",
         "symfony/mcp-bundle": "^0.6.0",
+        "symfony/mime": "8.0.*",
         "symfony/property-access": "8.0.*",
         "symfony/property-info": "8.0.*",
         "symfony/rate-limiter": "8.0.*",