Add service-level coverage for the Enhanced Library I Have This flow: copying an existing source file, writing the target album DB row, preserving source audio, inheriting album identity tags, and migrating older track tables that lack disc_number.
Move the existing-file missing-track import workflow out of web_server.py and into core/library/missing_track_import.py.
Keep the Flask route focused on request wiring and response formatting while the service handles staging copy, post-processing, album identity tag inheritance, DB upsert, and media-server sync.
