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/093_subscription_plans.sql

@@ -0,0 +1,18 @@
+CREATE TABLE IF NOT EXISTS subscription_plans (
+    id BIGSERIAL PRIMARY KEY,
+    group_id BIGINT NOT NULL,
+    name VARCHAR(100) NOT NULL,
+    description TEXT NOT NULL DEFAULT '',
+    price DECIMAL(20,2) NOT NULL,
+    original_price DECIMAL(20,2),
+    validity_days INT NOT NULL DEFAULT 30,
+    validity_unit VARCHAR(10) NOT NULL DEFAULT 'day',
+    features TEXT NOT NULL DEFAULT '',
+    product_name VARCHAR(100) NOT NULL DEFAULT '',
+    for_sale BOOLEAN NOT NULL DEFAULT TRUE,
+    sort_order INT NOT NULL DEFAULT 0,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+CREATE INDEX IF NOT EXISTS idx_subscription_plans_group_id ON subscription_plans(group_id);
+CREATE INDEX IF NOT EXISTS idx_subscription_plans_for_sale ON subscription_plans(for_sale);