feat(admin): 渠道管理 - 模型供应商支持渠道级 SOCKS5/HTTP 代理

- ModelChannel 新增 proxy_url 字段，v48 迁移自动加列
- outbound 新增 OutboundHTTPClientWithProxy，socks5/socks5h/http/https 均支持；
  目标主机名不做本地解析，直接交给代理服务器远端解析，本地开启 TUN/VPN 时仍可用
- 系统代理、Provider 出站、管理员模型测试路径均使用渠道级代理
- 渠道管理表单新增"代理地址"输入框，留空回退全局代理

backend/internal/database/migrations.go

@@ -11,7 +11,7 @@ import (
 	"gorm.io/gorm"
 )
 
-const CurrentSchemaVersion int64 = 47
+const CurrentSchemaVersion int64 = 48
 
 const baselineSchemaChecksum = "sha256:open-ai-canvas-schema-v1-20260830"
 const schemaMigrationAppliedAtIndexChecksum = "sha256:schema-migrations-applied-at-index-v2-20260830"
@@ -151,6 +151,12 @@ var schemaMigrations = []migration{
 	}},
 	{version: 46, name: "skill_curation", checksum: "sha256:skill-curation-v46-20261005", apply: migrateSkillCuration},
 	{version: 47, name: "skill_curation_roots", checksum: "sha256:skill-curation-roots-v47-20261005", apply: migrateSkillCurationRoots},
+	{version: 48, name: "channel_proxy_url", checksum: "sha256:channel-proxy-url-v48-20261009", apply: func(tx *gorm.DB) error {
+		if !tx.Migrator().HasColumn(&model.ModelChannel{}, "ProxyURL") {
+			return tx.Migrator().AddColumn(&model.ModelChannel{}, "ProxyURL")
+		}
+		return nil
+	}},
 }
 
 func migratePrefixedIDSequenceReconcile(tx *gorm.DB) error {