Extract and store outbound links (backlinks) during content fetch

Adds a backlinks table: one row per <a href> on a captured page that
points at a different domain (same-site nav links are skipped).
Piggybacks on the existing --with-metadata WARC fetch/DOM-parse in
WarcRecordFetcher rather than a separate command/job, since extracting
links needs the same HTML already being downloaded for
title/description/content — no extra requests.

WarcRecordFetcher gains href resolution (absolute, protocol-relative,
root-relative, query-only, and plain relative hrefs) against the
page's own URL. CommonCrawlIndexService::saveBacklinks() replaces a
page's link set wholesale on each (re)fetch, so reruns stay
idempotent. Site gets outboundLinks()/inboundLinks() relations, the
latter keyed by domain string since a link's target site may not be
indexed yet.

Verified against real .es data: 781 backlinks extracted across 205
distinct target domains (social platforms, Trustpilot, genuine
inter-site links with correct anchor text), and a rerun of the same
fetch left the count unchanged.

app/Models/Backlink.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class Backlink extends Model
+{
+    protected $fillable = [
+        'site_id',
+        'site_page_id',
+        'target_url',
+        'target_url_hash',
+        'target_domain',
+        'anchor_text',
+        'rel',
+    ];
+
+    public function site(): BelongsTo
+    {
+        return $this->belongsTo(Site::class);
+    }
+
+    public function sitePage(): BelongsTo
+    {
+        return $this->belongsTo(SitePage::class);
+    }
+}