Part 1 stopped existing full dates being destroyed; this adds first-class support
for full release dates so they can be set + persisted instead of truncated to a
year at the DB layer.
- Schema: new nullable `release_date TEXT` on the albums table (idempotent
ALTER-ADD-COLUMN repair on startup + the live CREATE). NULL = year-only, every
reader falls back to albums.year, so it ships safe/dormant.
- Tag writer: write_tags_to_file + build_tag_diff prefer db_data['release_date']
(the full date) over the year int; _date_to_write writes the full date. When
there's no release_date it's exactly Part-1 behavior (year, preserving an
equally-specific existing file date).
- Retag read path: SELECT al.release_date in the tag-preview/write queries and
thread it into _build_library_tag_db_data.
- Manual edit: release_date added to ALBUM_EDITABLE_FIELDS + a "Release Date"
field (YYYY-MM-DD, validated client-side) in the album editor; the artist-album
query returns it so existing values show. User-set dates are authoritative.
- Enrichment: Spotify + iTunes workers store the source's full release_date
(YYYY-MM / YYYY-MM-DD) when present, only when empty — never clobbering a
manual value.
Tests: writer uses release_date over year + overrides an existing file date;
falls back to year when absent; diff compares the full date. Migration verified
idempotent + enrichment no-clobber. 1435 tag/retag/db/library tests pass.
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