Steps 2+3 of typed metadata migration. Two album-info builders now
route through Album.from_<source>_dict() when caller passes a
known source:
- _build_album_info (album-tracks lookups)
- _build_single_import_context_payload (single-track import context)
Legacy duck-typing stays as fallback for unknown source, non-dict
input, or converter errors. Pure additive — existing callers
without source kwarg unchanged.
- split metadata lookup logic into core/metadata/*
- keep core/metadata_service.py as the legacy barrel
- update tests and artist-detail code to patch concrete modules
