feat: Audiobookshelf plugin architecture for Library Manager

[deucebucket]

Overview

Architecture scoping for an Audiobookshelf (ABS) plugin that exposes Library Manager as a metadata agent. LM already has abs_client.py for pulling data FROM ABS — this is about making LM available TO ABS as a metadata provider.

Goals

• ABS users can install LM as a metadata source plugin
• When ABS needs book metadata, it queries LM (which queries Skaldleita/BookDB + AI verification)
• Leverage LM's multi-source consensus system (Precog) for higher accuracy than single-source lookups
• Support audio fingerprinting for identification of untagged audiobooks

Architecture

See docs/abs-plugin-architecture.md for full writeup.

Key Integration Points

• ABS metadata provider API contract
• LM API endpoints needed for ABS queries
• Authentication between ABS and LM
• Confidence scoring translation (LM 0-100 → ABS format)
• Audio clip forwarding for Skaldleita Whisper identification

[deucebucket]

Correction: this issue was closed in error and has been reopened. PR #263 delivered only the architecture document; that is a prerequisite, not the feature. #257 remains open until Audiobookshelf can install/select Library Manager as a metadata provider during Match/Quick Match. The intended chain is Audiobookshelf -> Library Manager provider adapter -> Library Manager consensus/pipeline -> configured backends, including a user-configured local or hosted Skaldleita instance for metadata and audio identification. Library Manager owns ABS contract translation, confidence handling, and backend selection; Skaldleita remains a backend and is not treated as users.local or exposed directly to ABS. Completion requires end-to-end validation of that full chain.