Add file visibility (PRIVATE/PUBLIC) + public files API

Adds a visibility enum to files with PRIVATE (default) and PUBLIC states.
PUBLIC files are accessible via an unauthenticated /api/files/v1/{fileID}
endpoint that redirects to a presigned S3 URL. Introduces pkg/file service
to manage file operations. Logo uploads (trust centers, organizations,
frameworks, references) are marked PUBLIC; other files are PRIVATE.
Includes database migration and backfill for existing logos.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/esign/completion_certificate_worker.go

@@ -271,6 +271,7 @@ func (w *CompletionCertificateWorker) generateCertificate(
 		MimeType:       "application/pdf",
 		FileName:       certificateFilename,
 		FileKey:        uuid.MustNewV4().String(),
+		Visibility:     coredata.FileVisibilityPrivate,
 		CreatedAt:      time.Now(),
 		UpdatedAt:      time.Now(),
 	}