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).

supa_schema.sql

@@ -346,3 +346,78 @@ FOR UPDATE USING (
 -- FROM auth.users
 -- ON CONFLICT (user_id) DO UPDATE SET balance = EXCLUDED.balance
 -- WHERE user_credits.balance = 0;
+
+-- ============================================
+-- Subscription Plans System
+-- ============================================
+
+-- Subscription plans table
+CREATE TABLE IF NOT EXISTS subscription_plans (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    name TEXT NOT NULL UNIQUE,
+    display_name TEXT NOT NULL,
+    monthly_credits INTEGER NOT NULL,
+    price DECIMAL(10, 2) DEFAULT 0,
+    description TEXT,
+    is_active BOOLEAN DEFAULT true,
+    sort_order INTEGER DEFAULT 0,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT timezone('Europe/Berlin'::text, now()),
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT timezone('Europe/Berlin'::text, now())
+);
+
+CREATE INDEX idx_subscription_plans_active ON subscription_plans(is_active);
+CREATE INDEX idx_subscription_plans_sort_order ON subscription_plans(sort_order);
+
+-- User subscriptions table
+CREATE TABLE IF NOT EXISTS user_subscriptions (
+    user_id UUID PRIMARY KEY REFERENCES auth.users(id) ON DELETE CASCADE,
+    plan_id UUID REFERENCES subscription_plans(id) ON DELETE SET NULL,
+    started_at TIMESTAMP WITH TIME ZONE DEFAULT timezone('Europe/Berlin'::text, now()),
+    last_credit_grant_at TIMESTAMP WITH TIME ZONE DEFAULT timezone('Europe/Berlin'::text, now()),
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT timezone('Europe/Berlin'::text, now())
+);
+
+CREATE INDEX idx_user_subscriptions_plan_id ON user_subscriptions(plan_id);
+
+-- Enable RLS
+ALTER TABLE subscription_plans ENABLE ROW LEVEL SECURITY;
+ALTER TABLE user_subscriptions ENABLE ROW LEVEL SECURITY;
+
+-- Policies for subscription_plans (everyone can view active plans)
+CREATE POLICY "Anyone can view active plans" ON subscription_plans 
+FOR SELECT USING (is_active = true);
+
+CREATE POLICY "Admins can manage plans" ON subscription_plans 
+FOR ALL USING (
+  auth.uid() IN (SELECT user_id FROM admin_users)
+);
+
+-- Policies for user_subscriptions
+CREATE POLICY "Users can view their own subscription" ON user_subscriptions 
+FOR SELECT USING (auth.uid() = user_id);
+
+CREATE POLICY "Admins can view all subscriptions" ON user_subscriptions 
+FOR SELECT USING (
+  auth.uid() IN (SELECT user_id FROM admin_users)
+);
+
+CREATE POLICY "Admins can manage subscriptions" ON user_subscriptions 
+FOR ALL USING (
+  auth.uid() IN (SELECT user_id FROM admin_users)
+);
+
+-- Insert default plans
+INSERT INTO subscription_plans (name, display_name, monthly_credits, price, description, sort_order) VALUES
+('starter', 'Starter', 10, 0.00, 'Perfect for occasional use', 1),
+('bronze', 'Bronze', 50, 4.99, 'Great for regular users', 2),
+('silver', 'Silver', 100, 8.99, 'Best value for power users', 3),
+('gold', 'Gold', 250, 19.99, 'Unlimited searches for professionals', 4)
+ON CONFLICT (name) DO NOTHING;
+
+-- Set all existing users to Starter plan
+INSERT INTO user_subscriptions (user_id, plan_id)
+SELECT 
+    u.id,
+    (SELECT id FROM subscription_plans WHERE name = 'starter' LIMIT 1)
+FROM auth.users u
+ON CONFLICT (user_id) DO NOTHING;