feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/migrations/091_payment_audit_logs.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS payment_audit_logs (
+    id BIGSERIAL PRIMARY KEY,
+    order_id VARCHAR(64) NOT NULL,
+    action VARCHAR(50) NOT NULL,
+    detail TEXT NOT NULL DEFAULT '',
+    operator VARCHAR(100) NOT NULL DEFAULT 'system',
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+CREATE INDEX IF NOT EXISTS idx_payment_audit_logs_order_id ON payment_audit_logs(order_id);