feat(artists): show provider-neutral upcoming concerts

docs/artist_concerts.md

@@ -0,0 +1,36 @@
+# Artist concerts
+
+Any metadata extension can include an optional `concerts` list in `getArtist`
+or the `artist` object returned by `handleUrl`. The native metadata bridge
+preserves it as `artist_info.concerts` or `artist.concerts`, respectively.
+Older extensions may omit it; no concert button is shown for an empty list.
+
+```json
+{
+  "concerts": [{
+    "id": "event-123",
+    "location": "Example City, Region",
+    "venue": "Example Hall",
+    "start_at": "2026-10-07T01:00:00Z",
+    "time_zone": "America/New_York",
+    "url": "https://example.com/events/123"
+  }]
+}
+```
+
+- `location` and a valid ISO 8601 `start_at` are required. Date-only values
+  (`2026-10-06`) are supported when a time has not been announced.
+- For UTC timestamps, supply the venue's IANA `time_zone`; the example above
+  displays **October 6 at 21:00**, regardless of the listener's device time zone.
+  Explicit offsets are also supported. Without a recognized time zone, the app
+  preserves the wall-clock fields supplied in `start_at`.
+- `id`, `venue`, and `url` are optional. The app deduplicates by event ID (or
+  date/location/venue), orders events by date, and accepts at most 500 entries.
+- Only HTTP(S) event links can be opened, and only after tapping an event.
+- Providers should return upcoming events only. Failure to fetch this optional
+  information must not fail the artist's albums or top tracks.
+
+The badge opens a native schedule with artist identity, venue-local date tiles,
+and event details. Labels are provider-neutral in both application themes.
+Concert metadata is supplied with the artist response, so the schedule opens
+immediately without a second loading state or network request.