fix: serialize Stripe checkout provisioning

migrations/017_billing_checkout_invariants.sql

@@ -0,0 +1,59 @@
+DO $$
+BEGIN
+  IF EXISTS (
+    SELECT 1
+    FROM users
+    WHERE billing_customer_id IS NOT NULL AND billing_customer_id <> ''
+    GROUP BY billing_customer_id
+    HAVING COUNT(*) > 1
+  ) THEN
+    RAISE EXCEPTION 'duplicate users.billing_customer_id values require Stripe reconciliation before migration 017';
+  END IF;
+
+  IF EXISTS (
+    SELECT 1
+    FROM subscriptions
+    WHERE provider = 'stripe' AND status <> 'canceled'
+    GROUP BY user_id
+    HAVING COUNT(*) > 1
+  ) THEN
+    RAISE EXCEPTION 'multiple open Stripe subscriptions per user require reconciliation before migration 017';
+  END IF;
+END $$;
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_users_billing_customer_unique
+  ON users(billing_customer_id)
+  WHERE billing_customer_id IS NOT NULL AND billing_customer_id <> '';
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_subscriptions_user_open_stripe_unique
+  ON subscriptions(user_id)
+  WHERE provider = 'stripe' AND status <> 'canceled';
+
+CREATE TABLE IF NOT EXISTS billing_checkout_sessions (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  plan_id UUID NOT NULL REFERENCES plans(id),
+  stripe_customer_id VARCHAR(200),
+  stripe_session_id VARCHAR(200),
+  checkout_url TEXT,
+  status VARCHAR(20) NOT NULL DEFAULT 'pending',
+  expires_at TIMESTAMPTZ NOT NULL,
+  lease_owner UUID,
+  lease_until TIMESTAMPTZ,
+  error_message TEXT,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  completed_at TIMESTAMPTZ,
+  CONSTRAINT billing_checkout_sessions_status_check
+    CHECK (status IN ('pending', 'completed', 'expired', 'failed', 'canceled')),
+  CONSTRAINT billing_checkout_sessions_stripe_session_unique
+    UNIQUE (stripe_session_id)
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_billing_checkout_sessions_user_pending
+  ON billing_checkout_sessions(user_id)
+  WHERE status = 'pending';
+
+CREATE INDEX IF NOT EXISTS idx_billing_checkout_sessions_expiry
+  ON billing_checkout_sessions(expires_at)
+  WHERE status = 'pending';