Persist complete OPDS publication views and fallback on missing metadata

[panaC]

fixes #3918

Persist the complete IOpdsPublicationView in the local PublicationDocument when importing from an OPDS catalog.
Use the persisted OPDS metadata as a field-by-field fallback when creating a local PublicationView.

• Local publication metadata remains authoritative.
• OPDS metadata fills only missing or empty values.
• Supports:
  • Titles
  • Authors and publishers
  • Work identifier
  • Description
  • Languages and tags
  • Publication date
  • Accessibility metadata
  • Duration and track count
• Applies to regular and unavailable/minimal publication views.
• Treats the legacy "-" title placeholder as missing.
• Keeps existing serialized OPDS data and acquisition identifiers for compatibility.
• Publications imported outside OPDS remain unchanged.

[panaC]

For example with this OPDS feed:

[panaC]