feat(billing): add Feishu notification on payment success

[GenerQAQ]

Why we need this PR?

Add async Feishu notifications when payments succeed or new subscriptions activate, so the team gets real-time payment alerts with user and usage context.

Describe your solution

Added a sendPaymentFeishuNotification() helper in the stripe-webhook function that:

• Reads FEISHU_WEBHOOK_URL from env (silently skips if unset)
• Queries organization_usage for the org's current month metrics
• Fetches customer email from Stripe
• Sends a Feishu interactive card with payment info + usage summary
• Differentiates new subscriptions (green header) vs renewals (yellow header)
• Fire-and-forget: errors logged but never block the webhook response

Implementation Tasks

• Add sendPaymentFeishuNotification() helper function
• Call on customer.subscription.created/updated (new sub activation)
• Call on invoice.payment_succeeded (renewal/payment)

Impact Areas

• Dashboard

Checklist

• Open your pull request against the dev branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.