feat: implement subscription plan system with monthly credits

- Database schema:
  * subscription_plans table - stores plan tiers (Starter, Bronze, Silver, Gold)
  * user_subscriptions table - assigns users to plans
  * Default plans created (10, 50, 100, 250 credits/month)
  * All existing users assigned to Starter plan

- Subscription service (subscription-service.ts):
  * getAllPlans() - fetch all plans
  * getActivePlans() - fetch active plans for users
  * createPlan() - admin creates new plan
  * updatePlan() - admin edits plan
  * deletePlan() - admin removes plan
  * getUserSubscription() - get user's current plan
  * setUserPlan() - admin assigns user to plan
  * grantMonthlyCredits() - distribute credits to all users

- Plan management interface (/admin/plans):
  * Visual plan cards with credits, price, description
  * Create/Edit/Delete plans
  * Toggle active/inactive status
  * Sort order management
  * Grant monthly credits button (manual trigger)

- Features:
  * Monthly credit allocation based on plan
  * Prevents duplicate credit grants (tracks last_credit_grant_at)
  * Admin can manually trigger monthly credit distribution
  * Plans can be activated/deactivated
  * Custom pricing and credit amounts per plan

- UI:
  * Beautiful plan cards with color coding
  * Modal for create/edit with validation
  * Success/error messages
  * Manage Plans button in admin dashboard

Ready for future automation (cron job for monthly credits)
and payment integration (Stripe/PayPal).

src/app/admin/page.tsx

@@ -83,6 +83,12 @@ export default async function AdminPage() {
                         <p className="text-zinc-500 mt-1">API Usage Monitoring & Statistics</p>
                     </div>
                     <div className="flex gap-3">
+                        <Link
+                            href="/admin/plans"
+                            className="px-4 py-2 bg-purple-600 hover:bg-purple-700 text-white rounded-xl font-bold transition-colors"
+                        >
+                            Manage Plans
+                        </Link>
                         <Link
                             href="/admin/users"
                             className="px-4 py-2 bg-amber-600 hover:bg-amber-700 text-white rounded-xl font-bold transition-colors"