Add per-user favorites for artists and releases

[darious]

Summary

• add per-user artist and release favorites with a safe additive database migration
• expose favorite state and toggle endpoints in the API and surface it on artist and album pages
• add a favorites-only filter on the artists page and fix artist-page UI reactivity after toggling
• stabilize the dev frontend startup by isolating SvelteKit and Vite generated state in Docker volumes

Migration safety

• migration 028_user_favorites.sql is additive only
• uses CREATE TABLE IF NOT EXISTS and CREATE INDEX IF NOT EXISTS
• does not modify or delete existing users, artists, albums, or favorites data

Verification

• npm run check
• ./test.sh tests/auth/test_migration_028.py tests/api/test_favorites.py tests/api/test_library.py