fix(faq): correct PostgreSQL cast precedence in FAQ search

`metadata->'similar_questions'::text` parses as
`metadata -> ('similar_questions'::text)` because `::` binds tighter
than `->`, so the expression yields jsonb instead of text. ILIKE on
jsonb then fails with "operator does not exist: jsonb ~~* unknown",
returning 1007 Internal server error for `search_field=similar_questions`
and `search_field=answers`. The `standard_question` branch was unaffected
because it uses `->>` (text) directly.

Wrap the json access in parens so the cast applies to the extracted
value: `(metadata->'similar_questions')::text ILIKE ?`. The MySQL
branch uses JSON_EXTRACT and is unchanged.

Fixes #1264

internal/application/repository/chunk.go

@@ -193,14 +193,14 @@ func (r *chunkRepository) ListPagedChunksByKnowledgeID(
 			case "similar_questions":
 				// Search in similar_questions array of metadata
 				if isPostgres {
-					db = db.Where("metadata->'similar_questions'::text ILIKE ?", like)
+					db = db.Where("(metadata->'similar_questions')::text ILIKE ?", like)
 				} else {
 					db = db.Where("JSON_EXTRACT(metadata, '$.similar_questions') LIKE ?", like)
 				}
 			case "answers":
 				// Search in answers array of metadata
 				if isPostgres {
-					db = db.Where("metadata->'answers'::text ILIKE ?", like)
+					db = db.Where("(metadata->'answers')::text ILIKE ?", like)
 				} else {
 					db = db.Where("JSON_EXTRACT(metadata, '$.answers') LIKE ?", like)
 				}