Add non-blocking duplicate check when adding a record

[snowmonkeylab]

Summary

The backend groundwork for this already existed and was clearly built for it, just never wired up: store.find_by_artist_album()'s own docstring says "used to flag possible duplicates while shopping", and the search service already returns an exact_match field when queried with an "Artist - Album" string. No backend changes needed.

• Checks fire when selecting a Discogs search result (immediate) and while manually typing artist/album (debounced 500ms, matching the existing Discogs-search debounce pattern), guarded against out-of-order async responses via a request token.
• Add-flow only — editing an existing record doesn't check against itself.
• Shows a neutral, non-blocking notice under the Artist/Album fields — never disables "Add to Collection".
• Distinguishes an existing Collection match ("You already have this in your Collection.") from a Wish List match ("This is in your Wish List."). For the Wish List case, adds a "Move to Collection" action that updates the existing record instead of creating a duplicate — reusing the existing _saveRecord path (and explicitly clearing is_next_buy, since that flag only makes sense for wishlist items), so it inherits the already-fixed sensor-refresh behavior for free.

Test plan

• Verified store.find_by_artist_album() directly: case-insensitive matching, correctly distinguishes a Wish List match from a Collection match, no false positives.
• Confirmed the backend query-splitting logic (query.split(" - ", 1)) the card's "<artist> - <album>" query format relies on.
• Confirmed handle_update_record (used by the "Move to Collection" action) still calls _refresh_sensors on main.
• Deployed to a local Docker HA test instance; clean restart with no tracebacks.
• Click through in the browser: Discogs-selection trigger, manual-typing trigger, Collection-match wording, Wish-List-match wording + Move to Collection action, sensor counts updating after the move.

🤖 Generated with Claude Code