fix(import): decrypt owner-encrypted PDFs with pdfcpu; add imports bucket to minio-init

- parsePDF now attempts to strip encryption via pdfcpu (empty user password)
  before handing bytes to dslipak/pdf — fixes '256-bit encryption key' error
  on publisher PDFs that use owner-only encryption (copy/print restrictions)
- Add pdfcpu v0.11.1 as direct dependency (was already indirect)
- docker-compose.yml minio-init: add 'imports' and 'translations' buckets
  so a fresh deploy creates all required buckets

backend/go.mod

@@ -38,6 +38,7 @@ require (
 	github.com/minio/crc64nvme v1.1.1 // indirect
 	github.com/minio/md5-simd v1.1.2 // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
+	github.com/pdfcpu/pdfcpu v0.11.1 // indirect
 	github.com/philhofer/fwd v1.2.0 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect