Phase 3.1: OSS storage config management (MinIO/AliyunOSS/AWS S3)

Backend:
- Add OSSStorageConfig model (models.py)
- Create storage_admin.py API (9 endpoints): stats, CRUD, toggle, set-default, test connection
- Register route /app/api/admin/storage in main.py
- Create oss_storage_configs table in MySQL

Frontend:
- Add Storage.tsx admin page (713 lines) with stats, table, create/edit modal
- Register /admin/storage route in App.tsx
- Add nav item and icon in AdminLayout.tsx
- Add 9 API methods in api/index.ts

backend/python/app/models.py

@@ -979,3 +979,22 @@ class ExternalKBService(Base):
     status = Column(String(20), default="disconnected")
     created_at = Column(DateTime, default=datetime.now)
     updated_at = Column(DateTime, default=datetime.now, onupdate=datetime.now)
+
+class OSSStorageConfig(Base):
+    __tablename__ = "oss_storage_configs"
+
+    id = Column(String(36), primary_key=True, default=gen_uuid)
+    name = Column(String(200), nullable=False)
+    provider = Column(String(20), nullable=False)
+    endpoint = Column(String(500), nullable=False)
+    access_key = Column(String(255), nullable=False)
+    secret_key = Column(String(255), nullable=False)
+    bucket = Column(String(200), nullable=False)
+    region = Column(String(100), default="")
+    base_path = Column(String(300), default="")
+    is_default = Column(Boolean, default=False)
+    is_enabled = Column(Boolean, default=True)
+    status = Column(String(20), default="disconnected")
+    config = Column(JSON, default=None)
+    created_at = Column(DateTime, default=datetime.now)
+    updated_at = Column(DateTime, default=datetime.now, onupdate=datetime.now)