Store a SHA-256 hash of the webhook secret in a new webhook_secret_hash
column. FindAppByWebhookSecret now hashes the incoming secret and queries
by hash, eliminating the SQL string comparison timing side-channel.
- Add migration 005_add_webhook_secret_hash.sql
- Add database.HashWebhookSecret() helper
- Backfill existing secrets on startup
- Update App model to include WebhookSecretHash in all queries
- Update app creation to compute hash at insert time
- Add TestHashWebhookSecret unit test
- Update all test fixtures to set WebhookSecretHash
Closes#13
