fix: use strings.HasPrefix and delegate export to real cleanup method

- Replace manual string prefix check with strings.HasPrefix (idiomatic Go)
- Refactor CleanupCancelledDeploy export to call the real cleanupCancelledDeploy
  method instead of re-implementing the logic, so tests catch regressions
- Add nil-db guard in Deployment.Save and App.Save to prevent panics when
  models are used without a database connection (e.g. in test exports)
- Fix wsl_v5 lint issue (missing blank line before assignment)

internal/models/app.go

@@ -62,6 +62,10 @@ func NewApp(db *database.Database) *App {
 
 // Save inserts or updates the app in the database.
 func (a *App) Save(ctx context.Context) error {
+	if a.db == nil {
+		return fmt.Errorf("no database connection")
+	}
+
 	if a.exists(ctx) {
 		return a.update(ctx)
 	}

internal/models/deployment.go

@@ -57,6 +57,10 @@ func NewDeployment(db *database.Database) *Deployment {
 
 // Save inserts or updates the deployment in the database.
 func (d *Deployment) Save(ctx context.Context) error {
+	if d.db == nil {
+		return fmt.Errorf("no database connection")
+	}
+
 	if d.ID == 0 {
 		return d.insert(ctx)
 	}