Add complete implementation with tests:
- New route POST /admin/exchange/<id>/state/open-registration
- State validation (only from draft state)
- Success/error messages
- Authentication required
- Update exchange detail template with "Open Registration" button
- 8 comprehensive integration tests
All acceptance criteria met:
- "Open Registration" action available from Draft state
- Exchange state changes to "Registration Open"
- Registration link becomes active
- Participants can now access registration form
- Success message displayed
- Only admin can perform action
- Redirects to exchange detail after completion
Story: 3.1
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Co-Authored-By: Claude <noreply@anthropic.com>
Phil Skentelbery
cc865a85dc