feat: add Terms & Conditions checkbox to signup (refs #4)

- Add terms_agreements table for legal audit trail
- Create terms-agreement feature capsule with repository
- Modify signup to create terms agreement atomically
- Add checkbox with PDF link to SignupForm
- Capture IP, User-Agent, terms version, content hash
- Update CLAUDE.md documentation index

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/_system/migrations/run-all.ts

@@ -26,6 +26,7 @@ const MIGRATION_ORDER = [
   'features/backup',            // Admin backup feature; depends on update_updated_at_column()
   'features/notifications',     // Depends on maintenance and documents
   'features/user-profile',      // User profile management; independent
+  'features/terms-agreement',   // Terms & Conditions acceptance audit trail
 ];
 
 // Base directory where migrations are copied inside the image (set by Dockerfile)