refactor(internal): replace log.Fatal with panic, add timezone to DSN, add empty S3 bucket check

Replace log.Fatal in dev service init with panic for consistency. Add timezone=UTC to DAV connection DSN. Add IsEmpty() check for S3 dev bucket.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/s3/s3_dev.go

@@ -23,6 +23,7 @@ type Uploader interface {
 	Download(ctx context.Context, bucket, key string, w io.Writer) error
 	Delete(ctx context.Context, bucket, key string) error
 	GetURL(ctx context.Context, bucket, key string) (string, error)
+	HealthCheck(ctx context.Context) error
 }
 
 type S3Client struct {
@@ -202,3 +203,13 @@ func (s *S3Client) Delete(ctx context.Context, bucket, key string) error {
 func (s *S3Client) GetURL(ctx context.Context, bucket, key string) (string, error) {
 	return fmt.Sprintf("%s/%s/%s", s.publicURL, bucket, key), nil
 }
+
+func (s *S3Client) HealthCheck(ctx context.Context) error {
+	_, err := s.client.HeadBucket(ctx, &s3.HeadBucketInput{
+		Bucket: aws.String(s.bucket),
+	})
+	if err != nil {
+		return fmt.Errorf("S3 bucket %q is not accessible (check S3_PUBLIC_URL / RUSTFS_ENDPOINT config): %w", s.bucket, err)
+	}
+	return nil
+}