fix(nix): Remove deprecated max_retries from Prometheus config

- Remove queue_config.max_retries option from observability.nix
- Option deprecated/removed in recent NixOS/Prometheus versions
- Found by nix eval audit (T039.S3 pre-deployment validation)

Error: services.prometheus.remoteWrite."[...]".queue_config.max_retries' does not exist

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nix/modules/observability.nix

@@ -57,7 +57,7 @@ in
             max_shards = 20;
             min_backoff = "30ms";
             max_backoff = "10s";
-            max_retries = 10;
+            # max_retries removed - deprecated in recent Prometheus/NixOS versions
           };
         }
       ];