The download analysis already matches every track to a library row via
check_track_exists / manual match, then discarded the result. Keep it: each
analysis_results entry now carries matched_file_path + matched_track_id (the
owned file's real location, or None). Symmetrically, a completed download task
now records final_file_path (where the import landed).
Purely additive, no behavior change, no new matching, zero perf cost — just
stops throwing away what the pipeline already computed. This is the foundation
for playlist materialization: owned + downloaded tracks both report where their
real file is, so the folder can be built by name match, not source IDs.
