fix(search): trigram % operator escape 수정 (%% → %)

SQLAlchemy text() + asyncpg dialect에서 trigram operator 위치의 %%는
unescape 안 되어 'text %% unknown' 에러 발생. 단일 %로 변경.
ILIKE의 string literal 안의 %%는 PostgreSQL에서 두 wildcard로 동작했으나,
operator 위치는 escape 처리 경로가 다름.

app/services/search/retrieval_service.py

@@ -48,14 +48,14 @@ async def search_text(
             WITH candidates AS (
                 -- title trigram (idx_documents_title_trgm)
                 SELECT id FROM documents
-                WHERE deleted_at IS NULL AND title %% :q
+                WHERE deleted_at IS NULL AND title % :q
                 UNION
                 -- ai_summary trigram (idx_documents_ai_summary_trgm 부분 인덱스 매치)
                 SELECT id FROM documents
                 WHERE deleted_at IS NULL
                   AND ai_summary IS NOT NULL
                   AND length(ai_summary) > 0
-                  AND ai_summary %% :q
+                  AND ai_summary % :q
                 UNION
                 -- FTS 통합 인덱스 (idx_documents_fts_full)
                 SELECT id FROM documents