feat(M003/S03): play count column + data pipeline

Backend:
- GetAllTracksWithFullMetadata queries now select play_count and last_played
- mapTrackRow accepts and passes through PlayCount/LastPlayed
- PlayCount + LastPlayed added to library.Track struct
- sqlcgen Row types updated with new fields

Frontend:
- PlayCount + LastPlayed added to library.Track TypeScript model
- 'Plays' column added to track-list column definitions (60px, right-aligned, sortable)

Queries without play data (search, genre, album) pass 0/empty defaults.

frontend/src/components/track-list/columns.ts

@@ -186,6 +186,16 @@ export const COLUMN_DEFS: Record<string, ColumnDef> = {
         comparator: (a, b) =>
             compareNum(a.FileSize, b.FileSize),
     },
+    playCount: {
+        id: 'playCount',
+        label: 'Plays',
+        accessor: (t) =>
+            t.PlayCount ? String(t.PlayCount) : '',
+        defaultWidth: '60px',
+        align: 'right',
+        comparator: (a, b) =>
+            compareNum(a.PlayCount, b.PlayCount),
+    },
 };
 
 /**