fix: 混合布尔丢失 OR — boolean_operator="mixed" 引擎分支修复

boolean_operator 有三种值：and/or/mixed。引擎只处理了 "or"，
"mixed" 落到了 else（AND），导致 A OR B AND C 中 OR 完全丢失。

修复两处：
- field_combine: 用 or_ 当 string(mixed) 场景，因为各组内是 OR 关系
- term_conditions 合并: mixed 与 or 同逻辑，OR 合并所有正条件

backend/app/services/search_engine.py

@@ -613,7 +613,7 @@ class AdvancedSearchEngine:
         term_conditions: list = []
 
         # 1. 字段级搜索 [TI] [AB] [TIAB] [AU] [TA] [LA] [VI] [IP] [PG] [LID]
-        field_combine = or_ if pp.boolean_operator == "or" else and_
+        field_combine = or_ if pp.boolean_operator in ("or", "mixed") else and_
         field_map = {
             "title": pp.title_terms,
             "abstract": pp.abstract_terms,
@@ -949,10 +949,10 @@ class AdvancedSearchEngine:
 
         # 将 term_conditions 加入 conditions
         if term_conditions:
-            if pp.boolean_operator == "or":
+            if pp.boolean_operator in ("or", "mixed"):
                 from sqlalchemy.sql.elements import UnaryExpression
                 from sqlalchemy.sql import operators as _sa_ops
-                # OR 模式下 NOT 项应独立 AND（PubMed: A OR B NOT C = (A OR B) AND NOT C）
+                # OR/mixed 模式下 NOT 项应独立 AND（PubMed: A OR B NOT C = (A OR B) AND NOT C）
                 pos_conds = [c for c in term_conditions
                              if not (isinstance(c, UnaryExpression) and c.modifier == _sa_ops.inv)]
                 neg_conds = [c for c in term_conditions