feat: add admin config loader with multi-key support

Made-with: Cursor

backend/config/admins.example.json

@@ -0,0 +1,4 @@
+[
+  { "name": "Alice", "key": "change-me-alice-key" },
+  { "name": "Bob", "key": "change-me-bob-key" }
+]

backend/config/load-admins.js

@@ -0,0 +1,55 @@
+const fs = require('fs');
+const path = require('path');
+
+const DEFAULT_CONFIG_PATH = path.join(__dirname, 'admins.json');
+
+function loadAdmins() {
+  const configPath = process.env.ADMIN_CONFIG_PATH || DEFAULT_CONFIG_PATH;
+
+  if (fs.existsSync(configPath)) {
+    const raw = fs.readFileSync(configPath, 'utf-8');
+    const admins = JSON.parse(raw);
+
+    if (!Array.isArray(admins) || admins.length === 0) {
+      throw new Error(`Admin config at ${configPath} must be a non-empty array`);
+    }
+
+    const names = new Set();
+    const keys = new Set();
+
+    for (const admin of admins) {
+      if (!admin.name || !admin.key) {
+        throw new Error('Each admin must have a "name" and "key" property');
+      }
+      if (names.has(admin.name)) {
+        throw new Error(`Duplicate admin name: ${admin.name}`);
+      }
+      if (keys.has(admin.key)) {
+        throw new Error(`Duplicate admin key found`);
+      }
+      names.add(admin.name);
+      keys.add(admin.key);
+    }
+
+    console.log(`[Auth] Loaded ${admins.length} admin(s) from ${configPath}`);
+    return admins;
+  }
+
+  if (process.env.ADMIN_KEY) {
+    console.log('[Auth] No admins config file found, falling back to ADMIN_KEY env var');
+    return [{ name: 'Admin', key: process.env.ADMIN_KEY }];
+  }
+
+  throw new Error(
+    'No admin configuration found. Provide backend/config/admins.json or set ADMIN_KEY env var.'
+  );
+}
+
+const admins = loadAdmins();
+
+function findAdminByKey(key) {
+  const match = admins.find(a => a.key === key);
+  return match ? { name: match.name } : null;
+}
+
+module.exports = { findAdminByKey, admins };