fix(migration): asyncpg 다중 statement 분리

asyncpg는 prepared statement에 다중 SQL 불가.
COMMENT 제거하고 ALTER TABLE만 유지.

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

migrations/134_extract_meta.sql

@@ -1,2 +1 @@
-ALTER TABLE documents ADD COLUMN IF NOT EXISTS extract_meta JSONB DEFAULT '{}';
-COMMENT ON COLUMN documents.extract_meta IS 'OCR 판정/실행 메타데이터: ocr_attempted, ocr_reason, ocr_skip_reason, ocr_chars, pymupdf_chars, ocr_terminal';
+ALTER TABLE documents ADD COLUMN IF NOT EXISTS extract_meta JSONB DEFAULT '{}'