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/Site.php

@@ -20,4 +20,16 @@ class Site extends Model
     {
         return $this->hasMany(SitePage::class);
     }
+
+    /** Links this site's pages make out to other domains. */
+    public function outboundLinks(): HasMany
+    {
+        return $this->hasMany(Backlink::class);
+    }
+
+    /** Backlinks pointing at this site, i.e. rows recorded against its domain as the target. */
+    public function inboundLinks(): HasMany
+    {
+        return $this->hasMany(Backlink::class, 'target_domain', 'domain');
+    }
 }