test: add rollback error condition tests

Add tests for Rollback method error paths:
- No previous image available
- Empty previous image string
- App deployment lock held
- App lock already acquired

Relates to #71

internal/service/deploy/deploy.go

@@ -82,7 +82,7 @@ type deploymentLogWriter struct {
 	lineBuffer bytes.Buffer // buffer for incomplete lines
 	mu         sync.Mutex
 	done       chan struct{}
-	flushed    sync.WaitGroup // waits for flush goroutine to finish
+	flushed    sync.WaitGroup  // waits for flush goroutine to finish
 	flushCtx   context.Context //nolint:containedctx // needed for async flush goroutine
 }