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/app/app.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/google/uuid"
 	"github.com/oklog/ulid/v2"
+
 	"go.uber.org/fx"
 
 	"git.eeqj.de/sneak/upaas/internal/database"
@@ -82,6 +83,7 @@ func (svc *Service) CreateApp(
 	}
 
 	app.WebhookSecret = uuid.New().String()
+	app.WebhookSecretHash = database.HashWebhookSecret(app.WebhookSecret)
 	app.SSHPrivateKey = keyPair.PrivateKey
 	app.SSHPublicKey = keyPair.PublicKey
 	app.Status = models.AppStatusPending