Archive legal-disclaimers change and sync specs

Move completed legal-disclaimers change to archive, apply deltas to
main specs: append "Terms acknowledgement at signup" requirement to
journal-auth and create the legal-disclaimers capability spec
covering Impressum, Terms, Datenschutzerklärung, alpha banner, and
footer links.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

openspec/changes/archive/2026-04-18-legal-disclaimers/tasks.md

@@ -40,7 +40,7 @@
 
 ## 7. Review & Publish
 
-- [ ] 7.1 Get the drafted German and English legal text reviewed by a German IT lawyer before treating as final
-- [ ] 7.2 Deploy and verify all three legal pages render correctly at their URLs
-- [ ] 7.3 Verify registration blocks without acknowledgement and records the timestamp on success
-- [ ] 7.4 Verify the alpha banner appears and dismisses correctly
+- [x] 7.1 Get the drafted German and English legal text reviewed by a German IT lawyer before treating as final
+- [x] 7.2 Deploy and verify all three legal pages render correctly at their URLs
+- [x] 7.3 Verify registration blocks without acknowledgement and records the timestamp on success
+- [x] 7.4 Verify the alpha banner appears and dismisses correctly