fix(clickhouse): defer ephemeral event attributes

Author: carderne

internal-packages/clickhouse/schema/042_reduce_task_events_v2_storage_overhead.sql

@@ -2,11 +2,7 @@
 
 -- Full-text search is served outside the source event table. Keeping these
 -- indexes here adds work to every event insert and merge without serving reads.
--- attributes remains an insert input for attributes_text, but is no longer
--- stored. Writers must include attributes in an explicit insert column list
--- because implicit INSERT column lists exclude EPHEMERAL columns.
 ALTER TABLE trigger_dev.task_events_v2
-    MODIFY COLUMN attributes JSON EPHEMERAL,
     DROP INDEX IF EXISTS idx_attributes_text_search,
     DROP INDEX IF EXISTS idx_attributes_text,
     DROP INDEX IF EXISTS idx_message_text_search,
@@ -15,6 +11,6 @@ SETTINGS mutations_sync = 2, alter_sync = 2;
 
 -- +goose Down
 
--- Restoring the stored JSON column safely requires inspecting the live schema
--- and coordinating the writer rollback. Use a new forward migration instead.
+-- Restoring these indexes safely requires selecting the appropriate index
+-- definitions and materializing them in a new forward migration.
 SELECT throwIf(1, 'This migration cannot be rolled back automatically');