fix: use hashed webhook secrets for constant-time comparison

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

internal/service/webhook/webhook_test.go

@@ -91,6 +91,7 @@ func createTestApp(
 	app.Branch = branch
 	app.DockerfilePath = "Dockerfile"
 	app.WebhookSecret = "webhook-secret-123"
+	app.WebhookSecretHash = database.HashWebhookSecret(app.WebhookSecret)
 	app.SSHPrivateKey = "private-key"
 	app.SSHPublicKey = "public-key"
 	app.Status = models.AppStatusPending