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/specs/journal-auth/spec.md

@@ -11,3 +11,19 @@ The journal auth system SHALL store OAuth tokens for external services alongside
 - **WHEN** a user connects their Wahoo account
 - **THEN** access token, refresh token, expiry time, and Wahoo user ID are stored in the `wahoo_tokens` table
 - **AND** tokens are associated with the journal user ID
+
+### Requirement: Terms acknowledgement at signup
+The registration form SHALL require explicit acknowledgement of the Terms of Service before an account can be created.
+
+#### Scenario: Checkbox required
+- **WHEN** a user views the registration form
+- **THEN** they see a required checkbox labeled "I have read and agree to the Terms of Service, including that trails.cool is in alpha and my data may be reset"
+- **AND** the checkbox label links to the Terms page
+
+#### Scenario: Cannot submit without acknowledgement
+- **WHEN** a user attempts to register without checking the acknowledgement box
+- **THEN** the form blocks submission and shows a validation message
+
+#### Scenario: Acknowledgement recorded
+- **WHEN** a user successfully registers
+- **THEN** the current timestamp is stored in `users.terms_accepted_at`