admin: editable MB work IDs and alternate titles on song detail

[dprodger]

Summary

Adds inline editing to the admin song detail page for MusicBrainz work IDs and alternate titles.

• MB Work ID and Secondary MB Work ID are both shown and editable. Entering an ID runs a MusicBrainz work lookup (cached via MusicBrainzSearcher) that surfaces the id, title, and composer/writer/lyricist credits. Save is gated on a successful lookup, so an unverified ID can't be persisted — editing the field after a lookup re-disables Save. Slots that already hold a value get a Clear button.
• Alternate titles render under the song title as chips, with an add/remove editor that saves to songs.alt_titles.

Endpoints

• GET /admin/musicbrainz/work/<id>/lookup — UUID-validated; returns {id, title, composers:[{name,type}]}
• POST /admin/songs/<id>/mb-id — slot: primary|second, set or clear
• POST /admin/songs/<id>/alt-titles — replaces alt_titles (trim/de-dupe/empty→NULL)

CSRF is handled automatically by the existing static/js/admin.js fetch wrapper.

Verification

• admin.py compiles; all three routes register in the app URL map.
• MB lookup + composer extraction tested live against a real work ID.
• Template renders without Jinja errors; both MB rows expose editors; Add/Clear logic correct for empty vs. populated slots.
• Both inline <script> blocks pass node --check.

Live DB write tests were not run, since the local .env DATABASE_URL points at production.

Note

This screen edits the second_mb_id value only — it does not trigger a re-import of recordings from the newly-set work (the dual-work source_mb_work_id tagging). A "re-research" trigger could be wired in as a follow-up.

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