Match a paid Substack post's "Read more" link tolerantly (review item 8)

A trailing slash, tracking query, fragment, other scheme or host case on
either link no longer lets a paid post through the paid filter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/services/paywall_service.dart

@@ -54,8 +54,24 @@ class PaywallService {
   bool isPaid(Article a) =>
       paidTitle.hasMatch(a.title) || _isCutSubstackPost(a) || (_checks[a.link]?.paid ?? false);
 
-  static bool _isCutSubstackPost(Article a) =>
-      _substackCut.firstMatch(a.contentHtml)?.group(1) == a.link;
+  static bool _isCutSubstackPost(Article a) {
+    final readMore = _substackCut.firstMatch(a.contentHtml)?.group(1);
+    return readMore != null && _samePost(readMore, a.link);
+  }
+
+  /// Whether two links point at the same post. Only host and path count:
+  /// scheme, host case, a trailing slash, the query (tracking parameters)
+  /// and the fragment don't.
+  static bool _samePost(String a, String b) {
+    String? key(String link) {
+      final uri = Uri.tryParse(link.trim());
+      if (uri == null || uri.host.isEmpty) return null;
+      return '${uri.host.toLowerCase()}${uri.path.replaceFirst(RegExp(r'/+$'), '')}';
+    }
+
+    final keyA = key(a);
+    return keyA != null && keyA == key(b);
+  }
 
   bool isSponsored(Article a) => sponsoredTitle.hasMatch(a.title);