fix(migration): asyncpg 다중 statement 분리 (135/136)

a842c65 패턴과 동일. asyncpg는 prepared statement에 단일 SQL만 허용.
- 135: ALTER TABLE만, 세미콜론 제거
- 136: CREATE INDEX 별도 파일

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

migrations/135_ask_events_observability.sql

@@ -1,12 +1 @@
--- Phase E.1: ask_events 측정 필드 확장
--- answer_length/covered_aspects/missing_aspects/model_name/prompt_version 추가
--- E.3 (400→600자) 전후 비교 기준 + 단계 5 2-모델 체인 failure mode 분석 근거
-
-ALTER TABLE ask_events
-    ADD COLUMN IF NOT EXISTS answer_length INT,
-    ADD COLUMN IF NOT EXISTS covered_aspects JSONB,
-    ADD COLUMN IF NOT EXISTS missing_aspects JSONB,
-    ADD COLUMN IF NOT EXISTS model_name TEXT,
-    ADD COLUMN IF NOT EXISTS prompt_version TEXT;
-
-CREATE INDEX IF NOT EXISTS idx_ask_events_prompt_version ON ask_events(prompt_version);
+ALTER TABLE ask_events ADD COLUMN IF NOT EXISTS answer_length INT, ADD COLUMN IF NOT EXISTS covered_aspects JSONB, ADD COLUMN IF NOT EXISTS missing_aspects JSONB, ADD COLUMN IF NOT EXISTS model_name TEXT, ADD COLUMN IF NOT EXISTS prompt_version TEXT

migrations/136_ask_events_prompt_version_idx.sql

@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS idx_ask_events_prompt_version ON ask_events(prompt_version)