feat: restore Langfuse observability integration (#103)

- Add lib/langfuse.ts with client, trace input/output, telemetry config
- Add instrumentation.ts for OpenTelemetry setup with Langfuse span processor
- Add /api/log-save endpoint for logging diagram saves
- Add /api/log-feedback endpoint for thumbs up/down feedback
- Update chat route with sessionId tracking and telemetry
- Add feedback buttons (thumbs up/down) to chat messages
- Add sessionId tracking throughout the app
- Update env.example with Langfuse configuration
- Add @langfuse/client, @langfuse/otel, @langfuse/tracing, @opentelemetry/sdk-trace-node

contexts/diagram-context.tsx

@@ -16,7 +16,7 @@ interface DiagramContextType {
     drawioRef: React.Ref<DrawIoEmbedRef | null>;
     handleDiagramExport: (data: any) => void;
     clearDiagram: () => void;
-    saveDiagramToFile: (filename: string, format: ExportFormat) => void;
+    saveDiagramToFile: (filename: string, format: ExportFormat, sessionId?: string) => void;
 }
 
 const DiagramContext = createContext<DiagramContextType | undefined>(undefined);
@@ -107,7 +107,7 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
         setDiagramHistory([]);
     };
 
-    const saveDiagramToFile = (filename: string, format: ExportFormat) => {
+    const saveDiagramToFile = (filename: string, format: ExportFormat, sessionId?: string) => {
         if (!drawioRef.current) {
             console.warn("Draw.io editor not ready");
             return;
@@ -145,6 +145,9 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
                     extension = ".svg";
                 }
 
+                // Log save event to Langfuse (flags the trace)
+                logSaveToLangfuse(filename, format, sessionId);
+
                 // Handle download
                 let url: string;
                 if (typeof fileContent === "string" && fileContent.startsWith("data:")) {
@@ -174,6 +177,19 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
         drawioRef.current.exportDiagram({ format: drawioFormat });
     };
 
+    // Log save event to Langfuse (just flags the trace, doesn't send content)
+    const logSaveToLangfuse = async (filename: string, format: string, sessionId?: string) => {
+        try {
+            await fetch("/api/log-save", {
+                method: "POST",
+                headers: { "Content-Type": "application/json" },
+                body: JSON.stringify({ filename, format, sessionId }),
+            });
+        } catch (error) {
+            console.warn("Failed to log save to Langfuse:", error);
+        }
+    };
+
     return (
         <DiagramContext.Provider
             value={{