feat(tracing): add OpenTelemetry instrumentation to API gateway

- Add internal/tracing package with OTel tracer initialization
- HTTP middleware for server-side tracing (request/response attributes)
- Transport wrapper for client-side upstream call tracing
- Update proxy to use tracing transport
- Add OTEL_* env vars to k8s deployment

Traces flow: api-gateway -> otel-collector -> tempo -> grafana

internal/proxy/proxy.go

@@ -15,6 +15,7 @@ import (
 
 	"forgejo.riotpiao.com/rock/homelab-frontend/internal/config"
 	"forgejo.riotpiao.com/rock/homelab-frontend/internal/logging"
+	"forgejo.riotpiao.com/rock/homelab-frontend/internal/tracing"
 )
 
 // Handler is a reverse proxy that routes requests to configured upstreams.
@@ -325,8 +326,8 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// Set the director to apply path rewriting
 	proxy.Director = route.Director
 
-	// Use the connection-pooled transport
-	proxy.Transport = route.Transport
+	// Use the connection-pooled transport wrapped with tracing
+	proxy.Transport = tracing.NewTransport(route.Transport)
 
 	// Set error handler to log upstream errors
 	proxy.ErrorHandler = func(w http.ResponseWriter, r *http.Request, err error) {