feat(modules): add external module registration and defineModule support

- Add `./modules/define` export path pointing to `defineModule.js`
- Implement `registerExternalModules()` to handle modules passed via `zen.config.js`, with env var gating (`ZEN_MODULE_<NAME>=true`)
- Extract `buildAdminConfig()` helper to consolidate admin navigation/page config building
- Refactor `loadModuleConfig()` to use `buildAdminConfig()` and simplify public routes check
- Improve `initializeModuleTables()` to gracefully skip modules without `db.js` instead of erroring
- Update module discovery JSDoc to reflect external module registration support

src/modules/modules.actions.js

@@ -1,37 +1,36 @@
 /**
  * Module Actions Registry (Server-Side)
- * 
- * Import server actions for public pages (/zen/*) and dashboard.
- * Admin pages import actions directly from modules.
- * See modules.registry.js for full module creation guide.
- * 
- * Usage in consuming Next.js app:
- * ```
- * import { MODULE_ACTIONS, MODULE_DASHBOARD_ACTIONS } from '@hykocx/zen/modules/actions';
- * 
- * // Access module actions
- * const { getInvoiceByTokenAction } = MODULE_ACTIONS.invoice;
- * 
- * // Get dashboard stats
- * const stats = await MODULE_DASHBOARD_ACTIONS.invoice();
- * ```
+ *
+ * Static registry for internal module server actions.
+ * External modules registered via zen.config.js are resolved through the runtime registry.
+ *
+ * Usage:
+ *   import { getModuleActions } from '@hykocx/zen/modules/actions';
+ *   const { getInvoiceByToken } = getModuleActions('invoice');
  */
 
-// Register module actions (for public pages)
+import { getModule } from '../core/modules/registry.js';
+
+// Static actions for internal modules (add entries here for new internal modules)
 export const MODULE_ACTIONS = {
   posts: {},
 };
 
-// Register dashboard stats actions (for admin dashboard)
+// Static dashboard stats actions for internal modules
 export const MODULE_DASHBOARD_ACTIONS = {};
 
 /**
- * Get actions for a specific module
+ * Get actions for a specific module.
+ * Checks the static registry first, then the runtime registry for external modules.
+ *
  * @param {string} moduleName - Module name
  * @returns {Object} Module actions object or empty object
  */
 export function getModuleActions(moduleName) {
-  return MODULE_ACTIONS[moduleName] || {};
+  if (MODULE_ACTIONS[moduleName]) return MODULE_ACTIONS[moduleName];
+
+  // External modules declare their actions in their defineModule() config
+  return getModule(moduleName)?.actions ?? {};
 }
 
 /**