Fix NIP-11 caching and export streaming issues (v0.46.2)

- Fix Content-Type header being set on request instead of response
- Add Vary: Accept header to prevent browser/CDN caching NIP-11 for HTML
- Add periodic flushing during export for HTTP streaming (every 100 events)
- Add initial flush after headers to start streaming immediately
- Add X-Content-Type-Options: nosniff to prevent browser buffering

Files modified:
- app/handle-relayinfo.go: Fix header and add Vary: Accept
- app/server.go: Add initial flush and nosniff header for export
- pkg/database/export.go: Add periodic and final flushing for streaming

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

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

app/server.go

@@ -738,6 +738,12 @@ func (s *Server) handleExport(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set(
 		"Content-Disposition", "attachment; filename=\""+filename+"\"",
 	)
+	w.Header().Set("X-Content-Type-Options", "nosniff")
+
+	// Flush headers to start streaming immediately
+	if flusher, ok := w.(http.Flusher); ok {
+		flusher.Flush()
+	}
 
 	// Stream export
 	s.DB.Export(s.Ctx, w, pks...)