Update alert preferences

Upserts the user's preferences. If webhookUrl is set (or changed), a new HMAC-SHA256 signing secret is generated and returned once in the response as webhookSecret. It is never shown again — the user must copy it into their receiver. Subsequent saves without a URL change keep the existing secret; saving a different URL rotates it; clearing the URL clears the secret.

Webhook payload format (POST to the configured URL):

• Body: { type, id, timestamp, data } (JSON)
• Headers: X-Percher-Event, X-Percher-Delivery, X-Percher-Timestamp, X-Percher-Signature: sha256=<hex>
• Signature = HMAC-SHA256(secret, ${timestamp}.${body})
• Event types: deploy.failed, deploy.succeeded, app.crashed, app.unhealthy, app.recovered, domain.expiring, security.finding