Remove 3x-ui integration from the panel.

Drop API sync, multi-panel registry, and UI/protocol paths; keep DB columns only for compatibility with existing installs.

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

db/schema.sql

@@ -81,7 +81,7 @@ CREATE TABLE IF NOT EXISTS invite_links (
     max_uses INTEGER NOT NULL DEFAULT 1,
     used_count INTEGER NOT NULL DEFAULT 0,
     user_id UUID,
-    protocol TEXT NOT NULL DEFAULT 'xui',
+    protocol TEXT NOT NULL DEFAULT 'awg',
     server_id INTEGER NOT NULL DEFAULT 0,
     xui_inbound_id INTEGER NOT NULL DEFAULT 0,
     xui_panel_id TEXT NOT NULL DEFAULT '',