feat: add ADMIN_PWD and ALLOWED_GUILD_IDS support

- Add ADMIN_PWD and ALLOWED_GUILD_IDS env vars to config
- Extend PluginContext with adminPwd and allowedGuildIds
- Add adminAuth and guildFilter middleware for plugins
- Add /api/admin/login endpoint

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/core/plugin.ts

@@ -25,6 +25,8 @@ export interface Plugin {
 export interface PluginContext {
   client: Client;
   dataDir: string;
+  adminPwd: string;
+  allowedGuildIds: string[];
 }
 
 const loadedPlugins: Plugin[] = [];