Paywall awareness (#6) + replace-an-article (#7)

- paywall.py: conservative domain-level paywall detection (New Scientist,
  Nature, and common hard/soft paywalls). Never fetches pages — an honest hint.
- API: Article gains a 'paywalled' flag; the brief now leads with a gentle AND
  readable story (paywalled/charged stories stay in the five, just not first).
- New GET /api/replacement returns the next-best readable, unshown article
  (honors mood+prefs via the merged prefs param; gentle=true for hero swaps).
- UI: paywalled cards show 'May need a subscription'; a Replace / 'Find one I
  can read' action (always visible, while tuning actions stay tucked) swaps the
  card for a readable alternative, with a gentle notice when none remain.
- Tests: paywall detection + replacement behavior (77 total).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ideas.md

@@ -5,10 +5,10 @@
 
 * Ability to silence some categories temporarily (Maybe a user doesn't even want to see health-related articles, even good ones, so they're not reminded of an ongoing medical issue -- a way to avoid something purposely for a bit)  [done: pause topics/flavors in Boundaries]
 * Terms to avoid list (To filter even good news that you'd rather not hear about)  [done: avoid words/phrases in Boundaries]
-- Favorite/save articles
-- Soothing background colors/gradients per each category as you scroll.  Maybe a user preference.
+| Favorite/save articles  [tabled: needs accounts/logins for a larger footprint]
+| Soothing background colors/gradients per each category as you scroll.  Maybe a user preference.  [tabled: revisit deliberately; if done, whisper-quiet translucent tints, not neon]
 - I really like the coloring for the metadata highlighting in each card (The grading bubbles)
-- Some articles are behind paywalls.. what can we do?
-- After an article is read, can we add a refresh button to fetch a replacement for it in the list?
+* Some articles are behind paywalls.. what can we do?  [done: domain-level paywall flag + readable hero; future: down-weight paywalled sources like New Scientist]
+* After an article is read, can we add a refresh button to fetch a replacement for it in the list?  [done: "Find one I can read" / Replace swaps in the next readable article]
 * I want the top 5 to be tere, but I want the remaining categories to be hidden behing their selections.  So the main screen should show just the current highlights, and then the other articles should only be visible when in that category.  [done]
 * Title headings should be a little larger -- if you select Today, Today should look like a proper heading, bold and beautiful.  Switching to Wondow should show "Wonder" all nice and whatnot.  [done]