docs: ABS plugin architecture scoping document (#257)

[deucebucket]

Summary

• Adds docs/ABS-Plugin-Architecture.md scoping Library Manager as an Audiobookshelf metadata provider plugin
• Covers three architecture options (metadata provider, background enrichment, hybrid) with recommendation
• Maps LM→ABS field translations, confidence scoring, authentication, security considerations
• Defines 4 implementation phases

Documentation only — no code changes.

Closes #257