feat: implement comprehensive credits management system

- Database schema:
  * Extended user_credits table with daily_limit, API costs, last_reset_at
  * Created credit_transactions table for full audit trail
  * Added RLS policies for secure access control

- Core services:
  * credit-service.ts - balance checking, deduction, addition, transaction history
  * admin-credit-service.ts - admin controls for managing users and credits

- API integration:
  * Integrated credit checking into discover-whiskybase.ts
  * Credits deducted after successful API calls
  * Insufficient credits error handling

- Admin interface:
  * /admin/users page with user management
  * Statistics dashboard (total users, credits in circulation, usage)
  * Interactive user table with search
  * Edit modal for credit adjustment and settings
  * Per-user daily limits and API cost configuration

- Features:
  * Automatic credit initialization (100 credits for new users)
  * Credit transaction logging with balance_after tracking
  * Admin can add/remove credits with reason
  * Admin can set custom daily limits per user
  * Admin can set custom API costs per user
  * Low credit warnings (< 10 credits)
  * Full transaction history

- User experience:
  * Credits checked before API calls
  * Clear error messages for insufficient credits
  * Graceful handling of credit deduction failures

System is ready for future enhancements like credit packages,
auto-recharge, and payment integration.

src/app/admin/page.tsx

@@ -82,12 +82,20 @@ export default async function AdminPage() {
                         <h1 className="text-3xl font-black text-zinc-900 dark:text-white tracking-tight">Admin Dashboard</h1>
                         <p className="text-zinc-500 mt-1">API Usage Monitoring & Statistics</p>
                     </div>
-                    <Link
-                        href="/"
-                        className="px-4 py-2 bg-zinc-900 dark:bg-zinc-100 text-white dark:text-zinc-900 rounded-xl font-bold hover:bg-zinc-800 transition-colors"
-                    >
-                        Back to App
-                    </Link>
+                    <div className="flex gap-3">
+                        <Link
+                            href="/admin/users"
+                            className="px-4 py-2 bg-amber-600 hover:bg-amber-700 text-white rounded-xl font-bold transition-colors"
+                        >
+                            Manage Users
+                        </Link>
+                        <Link
+                            href="/"
+                            className="px-4 py-2 bg-zinc-900 dark:bg-zinc-100 text-white dark:text-zinc-900 rounded-xl font-bold hover:bg-zinc-800 transition-colors"
+                        >
+                            Back to App
+                        </Link>
+                    </div>
                 </div>
 
                 {/* Global Stats Cards */}