Add tech-stack fingerprinting (server, powered-by, generator)

WarcRecordFetcher was splitting out and discarding the HTTP response
headers to get at the HTML body — now it keeps and parses them, so
Server/X-Powered-By headers get captured alongside the <meta
name="generator"> tag already sitting in the parsed DOM. No extra
requests: same WARC segment already fetched for --with-metadata.

Site copies these from its homepage capture, same as
title/description/keywords. New badges on the site detail page show
the detected platform (e.g. "Wix.com Website Builder") and server
signature at a glance.

Verified against real .es data: correctly detected Wix
("00padel.es"), nginx/Apache/Cloudflare server headers, and PHP
X-Powered-By headers across the batch.

app/Models/Site.php

@@ -14,6 +14,9 @@ class Site extends Model
         'title',
         'description',
         'keywords',
+        'server',
+        'powered_by',
+        'generator',
     ];
 
     public function pages(): HasMany

app/Models/SitePage.php

@@ -27,6 +27,9 @@ class SitePage extends Model
         'title',
         'description',
         'keywords',
+        'server',
+        'powered_by',
+        'generator',
         'content',
         'raw',
         'record_hash',