D-01: callback_target contract drift cleanup.
- Remove CallbackTarget from Event struct and Validate
- Remove CallbackTarget from PlatformInboundMeta
- Remove defaultCallbackTarget and assignment from builder
- Remove callback_target column from INSERT/SELECT/dead_letter SQL
- Clean up all test literals and assertions
DB migration left untouched; column remains empty until a future schema
cleanup migration.
22384d155b