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/models/models_test.go

@@ -297,6 +297,7 @@ func TestAllApps(t *testing.T) {
 			app.Branch = testBranch
 			app.DockerfilePath = "Dockerfile"
 			app.WebhookSecret = "secret-" + strconv.Itoa(idx)
+			app.WebhookSecretHash = database.HashWebhookSecret(app.WebhookSecret)
 			app.SSHPrivateKey = "private"
 			app.SSHPublicKey = "public"
 
@@ -791,6 +792,7 @@ func createTestApp(t *testing.T, testDB *database.Database) *models.App {
 	app.Branch = testBranch
 	app.DockerfilePath = "Dockerfile"
 	app.WebhookSecret = "secret-" + t.Name()
+	app.WebhookSecretHash = database.HashWebhookSecret(app.WebhookSecret)
 	app.SSHPrivateKey = "private"
 	app.SSHPublicKey = "public"