refactor: remove Langfuse observability integration

- Delete lib/langfuse.ts, instrumentation.ts
- Remove API routes: log-save, log-feedback
- Remove feedback buttons (thumbs up/down) from chat
- Remove sessionId tracking throughout codebase
- Remove @langfuse/*, @opentelemetry dependencies
- Clean up env.example

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, sessionId?: string) => void;
+    saveDiagramToFile: (filename: string, format: ExportFormat) => 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, sessionId?: string) => {
+    const saveDiagramToFile = (filename: string, format: ExportFormat) => {
         if (!drawioRef.current) {
             console.warn("Draw.io editor not ready");
             return;
@@ -145,9 +145,6 @@ 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:")) {
@@ -177,19 +174,6 @@ 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={{