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.

database/migrations/2026_07_22_221213_add_tech_stack_columns.php

@@ -0,0 +1,42 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Server/X-Powered-By (from HTTP response headers, already downloaded alongside the
+     * HTML but previously discarded) and generator (from <meta name="generator">) — cheap
+     * tech-stack signals captured for free during the existing --with-metadata WARC fetch.
+     */
+    public function up(): void
+    {
+        Schema::table('site_pages', function (Blueprint $table) {
+            $table->string('server')->nullable()->after('keywords');
+            $table->string('powered_by')->nullable()->after('server');
+            $table->string('generator')->nullable()->after('powered_by');
+        });
+
+        Schema::table('sites', function (Blueprint $table) {
+            $table->string('server')->nullable()->after('keywords');
+            $table->string('powered_by')->nullable()->after('server');
+            $table->string('generator')->nullable()->after('powered_by');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('site_pages', function (Blueprint $table) {
+            $table->dropColumn(['server', 'powered_by', 'generator']);
+        });
+
+        Schema::table('sites', function (Blueprint $table) {
+            $table->dropColumn(['server', 'powered_by', 'generator']);
+        });
+    }
+};