perf(clickhouse): add words index for log search

Add a word-based text index for log search while retaining the existing
substring index. The migration does not materialize existing data.

Mono-RevId: 4b75eb5059887d5f5f92b0dd7c18c7458e1bcc29

Author: carderne

internal-packages/clickhouse/schema/050_add_task_events_search_words_index.sql

@@ -0,0 +1,11 @@
+-- +goose Up
+-- The identity expression permits ngram coexistence. ClickHouse 26.2 uses it for skip
+-- pruning, while 26.4 also supports direct reads from the words index. Keep this expression
+-- after dropping the ngram index. Do not add a follow-up migration back to the raw column.
+ALTER TABLE trigger_dev.task_events_search_v2
+  ADD INDEX IF NOT EXISTS idx_search_words concat(search_text, '')
+    TYPE text(tokenizer = 'splitByNonAlpha');
+
+-- +goose Down
+ALTER TABLE trigger_dev.task_events_search_v2
+  DROP INDEX IF EXISTS idx_search_words;