docs(tests): replace catchup.rs test with documentation

The catchup sync mechanism (reconnection with since filter) is implemented
in src/sync/mod.rs handle_connect_or_reconnect(), but cannot be reliably
integration tested with current infrastructure:

- TestRelay uses in-memory database (events lost on stop)
- No way to force WebSocket disconnection without stopping relay
- Stopping syncing relay creates new instance (fresh sync, not catchup)

Convert the skeleton test file to comprehensive documentation explaining:
- How catchup sync works (since filter on reconnect)
- The 15-minute quick reconnect window logic
- Why integration testing is not feasible
- Alternative approaches that could enable testing
- Related tests that cover adjacent functionality

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