feat: implement Stories 5.2 & 5.3 - Magic Link Login and Participant Session

Implemented complete participant authentication flow with magic link login and session management.

Story 5.2 - Magic Link Login:
- Participants can click magic links to securely access their dashboard
- Single-use tokens that expire after 1 hour
- Session creation with participant_id, user_type, and exchange_id
- Error handling for expired, used, or invalid tokens
- Fixed timezone-aware datetime comparison for SQLite compatibility

Story 5.3 - Participant Session:
- Authenticated participants can access their exchange dashboard
- participant_required decorator protects participant-only routes
- Participants can only access their own exchange (403 for others)
- Logout functionality clears session and redirects appropriately
- Unauthenticated access returns 403 Forbidden

Technical changes:
- Added magic_login() route for token validation and session creation
- Added dashboard() route with exchange and participant data
- Added logout() route with smart redirect to request access page
- Added participant_required decorator for route protection
- Enhanced MagicToken.is_expired for timezone-naive datetime handling
- Added participant.logout to setup check exclusions
- Created templates: dashboard.html, magic_link_error.html, 403.html
- Comprehensive test coverage for all user flows

Acceptance Criteria Met:
✓ Valid magic links create authenticated sessions
✓ Invalid/expired/used tokens show appropriate errors
✓ Authenticated participants see their dashboard
✓ Participants cannot access other exchanges
✓ Unauthenticated users cannot access protected routes
✓ Logout clears session and provides feedback

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

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

src/app.py

@@ -141,6 +141,7 @@ def register_setup_check(app: Flask) -> None:
             "participant.request_access",
             "participant.magic_login",
             "participant.dashboard",
+            "participant.logout",
         ]:
             return