feat(admin): add module settings to toggle trading feature

Introduce module_settings table to store global feature flags
Add admin UI to enable or disable the Trading module
Hide trading-related UI and skip data fetching when disabled

backend/db/schema.sql

@@ -219,6 +219,18 @@ INSERT INTO rate_limit_settings (id)
 VALUES (true)
 ON CONFLICT (id) DO NOTHING;
 
+CREATE TABLE IF NOT EXISTS module_settings (
+  id boolean PRIMARY KEY DEFAULT true CHECK (id = true),
+  trading_enabled boolean NOT NULL DEFAULT true,
+  updated_by_user_id integer REFERENCES users(id) ON DELETE SET NULL,
+  created_at timestamptz NOT NULL DEFAULT now(),
+  updated_at timestamptz NOT NULL DEFAULT now()
+);
+
+INSERT INTO module_settings (id)
+VALUES (true)
+ON CONFLICT (id) DO NOTHING;
+
 INSERT INTO permissions (key, name, description, category, system_permission)
 VALUES
   ('admin.access', 'Access admin', 'Open the management area.', 'Admin', true),