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/features/CLAUDE.md

@@ -16,6 +16,7 @@ Feature capsule directory. Each feature is 100% self-contained with api/, domain
 | `onboarding/` | User onboarding flow | First-time user setup |
 | `platform/` | Vehicle data and VIN decoding | Make/model lookup, VIN validation |
 | `stations/` | Gas station search and favorites | Google Maps integration, station data |
+| `terms-agreement/` | Terms & Conditions acceptance audit | Signup T&C, legal compliance |
 | `user-export/` | User data export | GDPR compliance, data portability |
 | `user-preferences/` | User preference management | User settings API |
 | `user-profile/` | User profile management | Profile CRUD, avatar handling |