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>

config/packages/security.yaml

@@ -22,6 +22,9 @@ security:
             pattern: ^/login_check
             stateless: true
             provider: app_user_provider
+            login_throttling:
+                max_attempts: 5
+                interval: '1 minute'
             json_login:
                 check_path: /login_check
                 username_path: username