feat: the admin Overview shows who is listening, downloads, the library and its disk, and the server's version, uptime and recent errors; a download waiting on you, a client not answering or a full disk count in its verdict (M504 #5627 #5628 #5629 #5630)

The version moves to internal/buildinfo, so /rest/ping reports the build
instead of "dev".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/queries/admin_status.sql

@@ -0,0 +1,54 @@
+-- The admin Overview's status sections (M504): who is listening, what is
+-- downloading, what the library holds. Each is one cheap read.
+
+-- name: ListListeningNow :many
+-- Plays in progress: open play events still inside their track's length,
+-- with a few minutes' grace for a pause. A client that never says a play
+-- ended would otherwise read as listening until the 24-hour sweep closes it.
+SELECT pe.started_at, pe.client_id, pe.device_class,
+       u.username,
+       t.id AS track_id, t.title AS track_title, t.duration_ms,
+       ar.name AS artist_name,
+       al.id AS album_id, al.title AS album_title
+  FROM play_events pe
+  JOIN users u    ON u.id = pe.user_id
+  JOIN tracks t   ON t.id = pe.track_id
+  JOIN albums al  ON al.id = t.album_id
+  JOIN artists ar ON ar.id = t.artist_id
+ WHERE pe.ended_at IS NULL
+   AND pe.started_at > now() - INTERVAL '24 hours'
+   AND pe.started_at + make_interval(secs => t.duration_ms / 1000.0) + INTERVAL '5 minutes' > now()
+ ORDER BY pe.started_at DESC
+ LIMIT 20;
+
+-- name: PlaysLastDay :one
+-- The last day's listening, for the line under Listening now.
+SELECT COUNT(*)::bigint                AS plays,
+       COUNT(DISTINCT user_id)::bigint AS listeners,
+       MAX(started_at)::timestamptz    AS last_at
+  FROM play_events
+ WHERE started_at > now() - INTERVAL '24 hours';
+
+-- name: DownloadsStatus :one
+-- Downloads at a glance: running, waiting on a person (and since when), and
+-- the last day's imports and give-ups.
+SELECT COUNT(*) FILTER (WHERE ended_at IS NULL AND import_state IN ('', 'downloading', 'waiting', 'importing'))::bigint AS running,
+       COUNT(*) FILTER (WHERE ended_at IS NULL AND import_state IN ('review', 'failed'))::bigint AS needs_you,
+       MIN(COALESCE(import_updated_at, grabbed_at)) FILTER (WHERE ended_at IS NULL AND import_state IN ('review', 'failed'))::timestamptz AS needs_you_since,
+       COUNT(*) FILTER (WHERE imported_at > now() - INTERVAL '24 hours')::bigint AS imported_day,
+       COUNT(*) FILTER (WHERE ended_at > now() - INTERVAL '24 hours'
+                          AND end_reason IN ('stalled', 'failed', 'gone', 'abandoned'))::bigint AS given_up_day
+  FROM grabs
+ WHERE ended_at IS NULL
+    OR ended_at > now() - INTERVAL '24 hours'
+    OR imported_at > now() - INTERVAL '24 hours';
+
+-- name: LibraryStatus :one
+-- What the library holds on disk: tracks whose files are present, the
+-- albums and artists they make, and their size.
+SELECT COUNT(*)::bigint                   AS tracks,
+       COUNT(DISTINCT album_id)::bigint   AS albums,
+       COUNT(DISTINCT artist_id)::bigint  AS artists,
+       COALESCE(SUM(file_size), 0)::bigint AS bytes
+  FROM tracks
+ WHERE missing_since IS NULL;