feat: add HTMX endpoint stubs for all admin modules

Templates use HTMX to load data dynamically via AJAX
Added 5 endpoint stubs (will implement with real data):

1. /admin/servers/matrix - Server status grid
2. /admin/players/table - Player list
3. /admin/grace/list - Grace period users (WORKING with real DB query)
4. /admin/audit/feed - Recent webhook events
5. /admin/roles/mismatches - Discord role diagnostics

Grace endpoint COMPLETE - queries PostgreSQL and shows users in grace period
Others return 'Coming Soon' placeholders

TESTING:
- Grace Period page should show real data
- Other pages show 'Coming Soon' instead of loading spinner

Signed-off-by: Claude (Chronicler #57) <claude@firefrostgaming.com>

services/arbiter-3.0/src/routes/admin.js

@@ -136,3 +136,104 @@ router.post('/mappings', isAdmin, express.json(), (req, res) => {
 });
 
 module.exports = router;
+
+// ==========================================
+// HTMX API ENDPOINTS (Return HTML fragments)
+// ==========================================
+
+// Servers Matrix Endpoint
+router.get('/servers/matrix', isAdmin, async (req, res) => {
+  // TODO: Query server status from Pterodactyl API
+  res.send(`
+    <div class="p-6 text-center text-gray-500 dark:text-gray-400">
+      <p class="text-lg font-medium mb-2">Server Matrix Coming Soon</p>
+      <p class="text-sm">Will display real-time status of all 13 Minecraft servers</p>
+    </div>
+  `);
+});
+
+// Players Table Endpoint  
+router.get('/players/table', isAdmin, async (req, res) => {
+  // TODO: Query subscriptions with Discord user data
+  res.send(`
+    <div class="p-6 text-center text-gray-500 dark:text-gray-400">
+      <p class="text-lg font-medium mb-2">Player Database Coming Soon</p>
+      <p class="text-sm">Will display all subscribers with search/filter</p>
+    </div>
+  `);
+});
+
+// Grace Period List Endpoint
+router.get('/grace/list', isAdmin, async (req, res) => {
+  const { Pool } = require('pg');
+  const pool = new Pool({
+    host: '127.0.0.1',
+    user: 'arbiter',
+    password: 'FireFrost2026!Arbiter',
+    database: 'arbiter_db'
+  });
+  
+  try {
+    const result = await pool.query(`
+      SELECT discord_id, tier_level, grace_period_started_at, grace_period_ends_at
+      FROM subscriptions 
+      WHERE status = 'grace_period'
+      ORDER BY grace_period_ends_at ASC
+    `);
+    
+    if (result.rows.length === 0) {
+      res.send(`
+        <div class="p-8 text-center text-gray-500 dark:text-gray-400">
+          <p class="text-lg">✅ No users in grace period!</p>
+          <p class="text-sm mt-2">All subscribers are current on payments</p>
+        </div>
+      `);
+    } else {
+      let html = '<div class="divide-y divide-gray-200 dark:divide-gray-700">';
+      result.rows.forEach(row => {
+        const endsAt = new Date(row.grace_period_ends_at);
+        const hoursLeft = Math.round((endsAt - new Date()) / (1000 * 60 * 60));
+        html += `
+          <div class="p-4 hover:bg-gray-50 dark:hover:bg-gray-800">
+            <div class="flex justify-between items-center">
+              <div>
+                <div class="font-medium dark:text-white">Discord ID: ${row.discord_id}</div>
+                <div class="text-sm text-gray-500">Tier ${row.tier_level}</div>
+              </div>
+              <div class="text-right">
+                <div class="text-yellow-600 dark:text-yellow-400 font-medium">${hoursLeft}h remaining</div>
+                <div class="text-xs text-gray-500">${endsAt.toLocaleString()}</div>
+              </div>
+            </div>
+          </div>
+        `;
+      });
+      html += '</div>';
+      res.send(html);
+    }
+  } catch (error) {
+    res.send(`<div class="p-6 text-red-600">Error: ${error.message}</div>`);
+  }
+});
+
+// Audit Log Feed Endpoint
+router.get('/audit/feed', isAdmin, async (req, res) => {
+  // TODO: Query webhook_events_processed table
+  res.send(`
+    <div class="p-6 text-center text-gray-500 dark:text-gray-400">
+      <p class="text-lg font-medium mb-2">Audit Log Coming Soon</p>
+      <p class="text-sm">Will display recent webhook events and role changes</p>
+    </div>
+  `);
+});
+
+// Role Mismatches Diagnostic Endpoint
+router.get('/roles/mismatches', isAdmin, async (req, res) => {
+  // TODO: Compare Discord roles vs database subscriptions
+  res.send(`
+    <div class="p-6 text-center text-gray-500 dark:text-gray-400">
+      <p class="text-lg font-medium mb-2">Role Diagnostics Coming Soon</p>
+      <p class="text-sm">Will scan Discord server and compare with database</p>
+    </div>
+  `);
+});