Add Gravatar and DNS Records collectors

Two pure-PHP collectors, no Python/venv/subprocess needed:
- GravatarRunner (email): checks gravatar.com/avatar/{md5}?d=404 for a
  200 vs 404. Scoped to that binary signal since Gravatar's profile
  JSON endpoint now requires API auth even for real, public profiles
  (confirmed against Gravatar's own creator's email).
- DnsRunner (domain): dns_get_record() for A/AAAA/MX/NS/TXT/CNAME. Raw
  payload is naturally keyed by record type, which the existing
  generic list-renderer on the investigation page turns into one
  collapsible section per type with no UI changes needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/Providers/AppServiceProvider.php

@@ -30,5 +30,7 @@ class AppServiceProvider extends ServiceProvider
         RateLimiter::for('osint-runner:sherlock', fn () => Limit::perMinute(10));
         RateLimiter::for('osint-runner:theharvester', fn () => Limit::perMinute(5));
         RateLimiter::for('osint-runner:holehe', fn () => Limit::perMinute(15));
+        RateLimiter::for('osint-runner:gravatar', fn () => Limit::perMinute(45));
+        RateLimiter::for('osint-runner:dns', fn () => Limit::perMinute(60));
     }
 }