chore(backend) : rate limiting, cache-control, remove twig, clean deps

- Add login_throttling on /login_check (5 attempts/min) with symfony/rate-limiter
- Add Cache-Control: public, max-age=86400 on avatar responses
- Remove symfony/twig-bundle (unused in API-only project)
- Remove unused dev deps: symfony/browser-kit, symfony/css-selector
- Rename API Platform title to "Lesstime API"

Tickets: T-010, T-016, T-022, T-024, T-025

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

composer.json

@@ -29,10 +29,10 @@
         "symfony/monolog-bundle": "^4.0",
         "symfony/property-access": "8.0.*",
         "symfony/property-info": "8.0.*",
+        "symfony/rate-limiter": "8.0.*",
         "symfony/runtime": "8.0.*",
         "symfony/security-bundle": "8.0.*",
         "symfony/serializer": "8.0.*",
-        "symfony/twig-bundle": "8.0.*",
         "symfony/validator": "8.0.*",
         "symfony/yaml": "8.0.*"
     },
@@ -91,8 +91,6 @@
     "require-dev": {
         "doctrine/doctrine-fixtures-bundle": "^4.3",
         "friendsofphp/php-cs-fixer": "^3.94",
-        "phpunit/phpunit": "^13.0",
-        "symfony/browser-kit": "8.0.*",
-        "symfony/css-selector": "8.0.*"
+        "phpunit/phpunit": "^13.0"
     }
 }