Add background PDF generation for published document versions

Move PDF generation from synchronous publish flow to a background polling
job. Published versions with file_id IS NULL are picked up by the job,
which generates the PDF, uploads to S3, and links the file. Export PDF
now serves stored files for published versions (with optional signature
page and watermark) and generates on the fly for drafts.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/coredata/errors.go

@@ -19,7 +19,8 @@ import (
 )
 
 var (
-	ErrResourceNotFound      = errors.New("resource not found")
-	ErrResourceAlreadyExists = errors.New("resource already exists")
-	ErrResourceInUse         = errors.New("resource is in use")
+	ErrResourceNotFound          = errors.New("resource not found")
+	ErrResourceAlreadyExists     = errors.New("resource already exists")
+	ErrResourceInUse             = errors.New("resource is in use")
+	ErrNoDocumentPDFJobAvailable = errors.New("no document PDF job available")
 )