feat: show node connection status and traffic stats in admin panel

Enable sing-box Clash/V2Ray APIs for per-user metrics, persist cumulative
traffic in SQLite, and refresh the dashboard every five seconds.

Co-authored-by: Cursor <cursoragent@cursor.com>

panel/db.py

@@ -58,6 +58,15 @@ def init_db(env: dict[str, str]) -> None:
             enabled INTEGER NOT NULL DEFAULT 1,
             created_at TEXT NOT NULL DEFAULT (datetime('now'))
         );
+        CREATE TABLE IF NOT EXISTS traffic_counters (
+            node_id INTEGER PRIMARY KEY,
+            upload_total INTEGER NOT NULL DEFAULT 0,
+            download_total INTEGER NOT NULL DEFAULT 0,
+            snapshot_upload INTEGER NOT NULL DEFAULT 0,
+            snapshot_download INTEGER NOT NULL DEFAULT 0,
+            updated_at TEXT,
+            FOREIGN KEY (node_id) REFERENCES nodes(id) ON DELETE CASCADE
+        );
         """
     )
 
@@ -80,6 +89,12 @@ def init_db(env: dict[str, str]) -> None:
             ("默认节点", uuid, hy2),
         )
 
+    for row in conn.execute("SELECT id FROM nodes").fetchall():
+        conn.execute(
+            "INSERT OR IGNORE INTO traffic_counters (node_id) VALUES (?)",
+            (row["id"],),
+        )
+
     conn.commit()
     conn.close()
 
@@ -115,6 +130,10 @@ def add_node(name: str) -> dict:
     )
     node_id = cur.lastrowid
     row = conn.execute("SELECT * FROM nodes WHERE id = ?", (node_id,)).fetchone()
+    conn.execute(
+        "INSERT OR IGNORE INTO traffic_counters (node_id) VALUES (?)",
+        (node_id,),
+    )
     conn.commit()
     conn.close()
     return dict(row)