ML-158: force single-line log output in production

Three-layer architecture:
- Logster v2 handles HTTP request and LiveView socket telemetry in logfmt format
- Custom formatter as safety net escaping any remaining embedded newlines
- Config flag (single_line_logging) gating Logster attachment to prod only

Dev environment retains multi-line format for readability.

mix.exs

@@ -172,6 +172,7 @@ defmodule MusicLibrary.MixProject do
 
       # Prod error/perf tooling
       {:error_tracker, "~> 0.9"},
+      {:logster, "~> 2.0.0-rc.5"},
       {:phoenix_live_dashboard, "~> 0.8.3"},
       {:recon, "~> 2.5"},
       {:telemetry_metrics, "~> 1.0"},