Improve record editing: allow changing artist and genre properly

[holden]

Problem

Editing a record currently has significant limitations around changing the artist and genre (and label) fields.

Issues Found

1. Artist autocomplete only returns artists already in the collection

• Api::ArtistsController scopes results to records: { user_id: COLLECTION_USER_ID } — so if you want to reassign a record to an artist not yet in your collection, the search returns nothing
• Same issue exists for labels (Api::LabelsController)

2. No way to create new artists, labels, or genres inline

• If the correct artist/label/genre doesn't exist in the database at all, there's no create-on-the-fly option in the edit form
• The form partial even has a comment noting this as a future phase: "Phase 2: Will add searchable dropdowns with create-on-the-fly for artists/labels"

3. Admin editing UX is buried

• On the admin record show page, artist/genre/label/format editing is hidden inside a collapsed <details> section ("Record associations") — easy to miss

Current State

The backend does permit these changes — record_params allows :artist_id, :label_id, :genre_id, :record_format_id — but the UI makes it impractical:

• Public edit form (/records/:id/edit): Has all fields but artist/label autocomplete is limited to existing collection entries
• Admin show page (/admin/records/:id): Has individual change buttons but hidden in collapsed section

Desired Behavior

• Artist autocomplete should search all artists in the database, not just those in the current collection
• Same for label autocomplete
• Add ability to create a new artist inline if search returns no results
• Add ability to create a new label inline if search returns no results
• Consider making the admin associations section more prominent or always visible
• Genre dropdown should include all genres (this already works correctly)

Files Involved

• app/controllers/api/artists_controller.rb — artist search scope
• app/controllers/api/labels_controller.rb — label search scope
• app/views/records/_form.html.erb — public edit form
• app/views/admin/records/show.html.erb — admin edit UI
• app/javascript/controllers/form_autocomplete_controller.js — autocomplete JS