feat: WeChat Pay V3 integration (fill credentials to activate)

Server:
- New lib/wxpay.js: native crypto RSA-SHA256 signing, JSAPI prepay,
  AES-256-GCM notify decryption, order query (no npm deps)
- New dao/payment-order.dao.js: createOrder, markPrepay,
  markPaidAndActivateSubscription (idempotent + transactional)
- New routes/payment.js: GET order status, POST order sync
- New routes/payment-notify.js: WeChat async callback handler with
  signature verification, amount/appid/mchid validation
- Modified subscription/purchase: auto-detects wxpay config, returns
  real payment_params or mock fallback
- Schema: payment_orders table with out_trade_no unique key
- app.js: express.raw() for notify path, payment routes mounted
- config.js: wxpay block with 7 env vars
- .env.example: all WeChat Pay fields documented
- .gitignore: certs/, *.pem, *.p12

Miniprogram:
- subscribe-plans doPurchase: calls real purchase API, falls back to
  mockPurchase only when server returns mock:true
- Added syncAndRedirect for post-payment order confirmation
- api.js: getPaymentOrder, syncPaymentOrder
- Removed "模拟支付"/"测试环境" from UI text

server/sql/schema.sql

@@ -159,6 +159,24 @@ CREATE TABLE IF NOT EXISTS firmware_files (
   KEY idx_firmware_version (version, status)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
 
+CREATE TABLE IF NOT EXISTS payment_orders (
+  order_id VARCHAR(64) NOT NULL COMMENT 'out_trade_no',
+  user_id BIGINT UNSIGNED NOT NULL,
+  plan VARCHAR(32) NOT NULL,
+  amount_fen INT UNSIGNED NOT NULL DEFAULT 0,
+  status VARCHAR(20) NOT NULL DEFAULT 'created' COMMENT 'created/paying/paid/closed/failed/refunded',
+  prepay_id VARCHAR(128) NOT NULL DEFAULT '',
+  transaction_id VARCHAR(64) NOT NULL DEFAULT '',
+  trade_state VARCHAR(32) NOT NULL DEFAULT '',
+  raw_notify_json JSON NULL,
+  paid_at DATETIME NULL,
+  created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  PRIMARY KEY (order_id),
+  KEY idx_payment_orders_user (user_id, status),
+  CONSTRAINT fk_payment_orders_user FOREIGN KEY (user_id) REFERENCES users (user_id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
 INSERT IGNORE INTO system_settings (setting_key, setting_value) VALUES
 ('system_name', '"光子美容仪后台"'),
 ('monthly_price', '99'),