refactor(api): validate and throttle feedback endpoint

- Validate content (required string, min:10, max:2000) in OtherController@feedback
- Register 'feedback' named rate limiter (3/min per user or IP) in RouteServiceProvider
- Apply throttle:feedback middleware to POST /api/feedback
- Forward to Discord with allowed_mentions.parse=[] and a 5s HTTP timeout

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/Providers/RouteServiceProvider.php

@@ -54,5 +54,9 @@ class RouteServiceProvider extends ServiceProvider
         RateLimiter::for('5', function (Request $request) {
             return Limit::perMinute(5)->by($request->user()?->id ?: $request->ip());
         });
+
+        RateLimiter::for('feedback', function (Request $request) {
+            return Limit::perMinute(3)->by($request->user()?->id ?: $request->ip());
+        });
     }
 }