Add multi-server 3x-ui registry with per-panel subscription URLs.

Admins can manage several 3x-ui panels by name and select which one issues invite/user configs via that server's /sub base URL.

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

db/schema.sql

@@ -42,6 +42,7 @@ CREATE TABLE IF NOT EXISTS user_connections (
     protocol TEXT NOT NULL DEFAULT '',
     client_id TEXT NOT NULL DEFAULT '',
     name TEXT NOT NULL DEFAULT '',
+    xui_panel_id TEXT NOT NULL DEFAULT '',
     created_at TIMESTAMPTZ,
     last_bytes BIGINT NOT NULL DEFAULT 0
 );
@@ -83,6 +84,7 @@ CREATE TABLE IF NOT EXISTS invite_links (
     protocol TEXT NOT NULL DEFAULT 'xui',
     server_id INTEGER NOT NULL DEFAULT 0,
     xui_inbound_id INTEGER NOT NULL DEFAULT 0,
+    xui_panel_id TEXT NOT NULL DEFAULT '',
     password_hash TEXT,
     expires_at TIMESTAMPTZ,
     duration_days INTEGER NOT NULL DEFAULT 0,