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>

symfony.lock

@@ -222,19 +222,6 @@
             "config/routes/security.yaml"
         ]
     },
-    "symfony/twig-bundle": {
-        "version": "8.0",
-        "recipe": {
-            "repo": "github.com/symfony/recipes",
-            "branch": "main",
-            "version": "6.4",
-            "ref": "cab5fd2a13a45c266d45a7d9337e28dee6272877"
-        },
-        "files": [
-            "config/packages/twig.yaml",
-            "templates/base.html.twig"
-        ]
-    },
     "symfony/uid": {
         "version": "8.0",
         "recipe": {