Hide paid Substack newsletter posts with the paid filter (#7)

Substack cuts paid-subscriber posts short in the feed and ends their
content with a "Read more" link to the post itself. PaywallService.isPaid
now recognizes that ending, so "hide paid articles" hides these posts in
Today and the Learn tab and counts them as hidden, without a page request.
Free posts that ask readers to subscribe or end with the subscribe widget
are not affected.

Tests use trimmed real items from the Interconnects and Ahead of AI feeds.

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

README.md

@@ -25,7 +25,9 @@ or write a daily briefing. It's meant to replace the n8n → Discord workflow.
 - **Paid articles and ads hidden**: heise+ and Golem `(g+)` are recognized by
   their title; SPIEGEL+ and t3n Pro by a marker in the article page, which the app
   checks once per new article in the background (it reads only until the marker,
-  and remembers results for 14 days). Sponsored posts (`Anzeige:`) are hidden too.
+  and remembers results for 14 days). Paid-subscriber posts from Substack newsletters
+  (Interconnects, Ahead of AI, …) are recognized in the feed itself, where Substack
+  cuts them off with a "Read more" link. Sponsored posts (`Anzeige:`) are hidden too.
   The feed header shows how many were hidden; both filters can be turned off.
 - **Ad and cookie-banner blocking** in the original-page view: requests to ~80
   ad/tracker hosts and the common consent managers (Sourcepoint, OneTrust,