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
- pass the selected manual match through singles import
- keep the import context source-aware so artist and album stay correct
- avoid treating non-Spotify IDs as wishlist Spotify IDs
- make wishlist logging and local variable names source-neutral
- Move import flow modules into a dedicated package
- Update app and test imports to the new namespace
- Group the import-focused tests under tests/imports
