fix(phase-16): forward X-Forwarded-For from EventProxyController so the API rate limiter partitions per client IP

Proxy chains any inbound XFF with the connection IP before relaying upstream; UseForwardedHeaders resolves it to the limiter's partition key. Documents the EventRepository first-play counter race (unique index is the backstop).

DeepDrftData/Repositories/EventRepository.cs

@@ -102,6 +102,12 @@ public class EventRepository
     // Bump the matching bucket column on the track's counter row, creating the row on first play. The
     // row is added to the change tracker but not saved here — the caller's SaveChanges/commit persists
     // it inside the same transaction as the event append.
+    //
+    // Race note: two concurrent first-plays of the same track can both reach this method, find no
+    // counter row, and both Add a new PlayCounter. The second SaveChanges will hit the unique index on
+    // (track_id) and throw, causing the outer transaction to roll back and the event to be dropped —
+    // no crash, no counter corruption. At the expected play volume this is an acceptable loss; the
+    // unique index is the integrity backstop.
     private async Task BumpCounterAsync(long trackId, PlayBucket bucket, CancellationToken ct)
     {
         var counter = await _context.PlayCounters.FirstOrDefaultAsync(c => c.TrackId == trackId, ct);