refactor(db): 抽出 GUID 类型并兼容 SQLite 的 JSONB 字段

- base_class.py 内置 GUID(TypeDecorator)，PostgreSQL 走原生 UUID，
  SQLite 走 CHAR(36)，UUID 在绑定/读取时统一转换。
- study_setup_config 与 study_setup_config_version 改用 JSONB().with_variant
  以便测试库使用 JSON 字段，生产仍保留 JSONB。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/db/base_class.py

@@ -1,5 +1,45 @@
+from __future__ import annotations
+
+import uuid
+
+from sqlalchemy import String, TypeDecorator
+from sqlalchemy.dialects.postgresql import UUID as PG_UUID
 from sqlalchemy.orm import DeclarativeBase
 
 
+class GUID(TypeDecorator):
+    """Platform-independent UUID type.
+
+    PostgreSQL keeps native UUID columns; SQLite test databases store canonical
+    UUID strings so bind/result behavior stays consistent.
+    """
+
+    impl = String
+    cache_ok = True
+
+    def load_dialect_impl(self, dialect):
+        if dialect.name == "sqlite":
+            return dialect.type_descriptor(String(36))
+        return dialect.type_descriptor(PG_UUID(as_uuid=True))
+
+    def process_bind_param(self, value, dialect):
+        if value is None:
+            return None
+        if dialect.name == "sqlite":
+            return str(value)
+        return value
+
+    def process_result_value(self, value, dialect):
+        if value is None:
+            return None
+        if dialect.name == "sqlite" and isinstance(value, str):
+            return uuid.UUID(value)
+        return value
+
+
 class Base(DeclarativeBase):
+    type_annotation_map = {
+        uuid.UUID: GUID(),
+    }
+
     pass