feat(proxy): 支持远程推送升级与proxy元数据上报

- aether-proxy 注册和心跳时上报 proxy_metadata（含版本号）
- 心跳 ACK 支持 upgrade_to 字段，proxy 收到后自动执行升级
- 重构 upgrade 逻辑，新增 perform_upgrade 用于远程触发的自动升级
- stream_handler 延迟统计改为仅记录连接建立延迟(DNS+TCP/TLS+TTFB)
- 后端新增 proxy_metadata 数据库字段和批量升级 API
- 远程配置支持下发 upgrade_to 版本指令
- 前端展示节点版本号，支持单节点和批量升级操作

alembic/versions/20260302_1300_1d2e3f4a5b6c_add_proxy_metadata_to_proxy_nodes.py

@@ -0,0 +1,46 @@
+"""add_proxy_metadata_to_proxy_nodes
+
+Revision ID: 1d2e3f4a5b6c
+Revises: f0c3a7b9d1e2
+Create Date: 2026-03-02 13:00:00.000000+00:00
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from sqlalchemy import inspect
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "1d2e3f4a5b6c"
+down_revision: str | None = "f0c3a7b9d1e2"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def _column_exists(table_name: str, column_name: str) -> bool:
+    bind = op.get_bind()
+    insp = inspect(bind)
+    columns = [c["name"] for c in insp.get_columns(table_name)]
+    return column_name in columns
+
+
+def upgrade() -> None:
+    if not _column_exists("proxy_nodes", "proxy_metadata"):
+        op.add_column(
+            "proxy_nodes",
+            sa.Column(
+                "proxy_metadata",
+                sa.JSON(),
+                nullable=True,
+                comment="aether-proxy 上报元数据（版本等）",
+            ),
+        )
+
+
+def downgrade() -> None:
+    if _column_exists("proxy_nodes", "proxy_metadata"):
+        op.drop_column("proxy_nodes", "proxy_metadata")