Contract 0004: drop the legacy event_outbox (service_outbox has carried every event since 2026-10-10) (#83)

ddiff --git a/migrations/0004_drop_event_outbox.sql b/migrations/0004_drop_event_outbox.sql
nnew file mode 100644
iindex 0000000..0911a60
---- /dev/null
++++ b/migrations/0004_drop_event_outbox.sql
@@@ -0,0 +1,5 @@
++-- phase: contract
++-- after: 0002
++-- The SDK outbox (service_outbox, migration 0002, 2026-10-10) has carried every Search event since; no code reads
++-- event_outbox, and none of its rows was pending when this was written (2026-10-23).
++DROP TABLE IF EXISTS event_outbox;