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.
Fixes 'invalid input syntax for type uuid' error when writing ticket
workflow audit logs. The audit Event.ID field was using fmt.Sprintf
with nanoseconds ('wf-%d') which doesn't match PostgreSQL's uuid type.
Also adds uuid import to ticket_workflow.go.
Verified: full chain webhook→assign→resolve→close produces 3 audit
logs correctly, no more 'invalid uuid' errors in logs.