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Motivation: platform-certificate mode is being phased out by WeChat (2024-10+,
newly-provisioned merchants already cannot download platform certificates at
all), and wxpay config errors currently surface only when an order is being
created — admins have no feedback at save time. Also, errors were returned as
natural-language strings, leaving the frontend no way to localize them.

Changes:

- backend/internal/payment/provider/wxpay.go
  - Replace fmt.Errorf with structured infraerrors.BadRequest errors:
    - WXPAY_CONFIG_MISSING_KEY    (metadata: key)
    - WXPAY_CONFIG_INVALID_KEY_LENGTH  (metadata: key, expected, actual)
    - WXPAY_CONFIG_INVALID_KEY    (metadata: key) for malformed PEMs
  - Parse privateKey and publicKey PEMs in NewWxpay so malformed keys fail
    at save time instead of at order creation.
  - Keep the pubkey verifier (NewSHA256WithRSAPubkeyVerifier) as the single
    supported verifier; no more loadKeyPair helper.

- backend/internal/service/payment_order.go invokeProvider
  - If CreateProvider or CreatePayment returns a structured ApplicationError,
    pass it through (optionally enriching metadata with provider/instance_id)
    instead of wrapping it as generic PAYMENT_GATEWAY_ERROR — so clients see
    the actual reason code (e.g. WXPAY_CONFIG_MISSING_KEY) and can localize.
  - Simplify a few messages (TOO_MANY_PENDING, DAILY_LIMIT_EXCEEDED,
    PAYMENT_GATEWAY_ERROR, NO_AVAILABLE_INSTANCE) to keyword form with
    metadata for template variables.

- backend/internal/service/payment_config_providers.go
  - New helper validateProviderConfig calls provider.CreateProvider at save
    time. Enabled instances are validated on both Create and Update so admins
    see config errors immediately in the dialog, not later at order creation.
  - Disabled instances are not validated (half-filled drafts are allowed).

- backend/internal/payment/provider/wxpay_test.go
  - Add generateTestKeyPair helper that produces valid RSA-2048 PKCS8/PKIX
    PEMs per test, used by the valid-config baseline (prior fake strings no
    longer pass the eager PEM check).
  - Cover each structured-error branch (missing/invalid-length/malformed PEM).

fix(payment): fix Alipay/Wxpay direct provider type mapping and enable cross-provider load balancing

Two issues fixed:

1. Alipay.SupportedTypes() returned ["alipay_direct"] and Wxpay returned
   ["wxpay_direct"], but the frontend sends payment_type="alipay"/"wxpay".
   The registry lookup failed with "payment method (alipay) is not
   configured". Fix: return the base types ["alipay"]/["wxpay"].

2. When multiple providers support the same payment type (e.g. EasyPay
   and Alipay direct both handle "alipay"), only the last-registered
   provider's instances were reachable — the registry mapped one type to
   one provider key, and SelectInstance queried by that single key.

   Fix: bypass the registry in invokeProvider and let SelectInstance
   query across all providers when providerKey is empty. The selected
   instance's own ProviderKey (now included in InstanceSelection) is
   used to create the correct provider, enabling true cross-provider
   load balancing.

Closes #1592

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