Stage 7 of the ABS-compat port (final). Implements MediaStore, ConfigProvider,
and event/recommender stub adapters, constructs the abs.Handler in
cmd/silo/main.go, and mounts it under /abs and /abs/api on silo's main
router. ABS-compatible Audiobookshelf clients can now hit silo
directly for login, library + items browse, file streaming, progress,
and Socket.io.
End-to-end smoke verified: /login returns 401, /abs/api/login returns
401, /abs/api/me returns 401, /abs/api/libraries returns 401,
/abs/socket.io/ returns 200 on polling handshake.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
