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

app/api/log-save/route.ts

@@ -1,65 +0,0 @@
-import { getLangfuseClient } from '@/lib/langfuse';
-import { randomUUID } from 'crypto';
-import { z } from 'zod';
-
-const saveSchema = z.object({
-  filename: z.string().min(1).max(255),
-  format: z.enum(['drawio', 'png', 'svg']),
-  sessionId: z.string().min(1).max(200).optional(),
-});
-
-export async function POST(req: Request) {
-  const langfuse = getLangfuseClient();
-  if (!langfuse) {
-    return Response.json({ success: true, logged: false });
-  }
-
-  // Validate input
-  let data;
-  try {
-    data = saveSchema.parse(await req.json());
-  } catch {
-    return Response.json({ success: false, error: 'Invalid input' }, { status: 400 });
-  }
-
-  const { filename, format, sessionId } = data;
-
-  try {
-    const timestamp = new Date().toISOString();
-
-    // Find the most recent chat trace for this session to attach the save flag
-    const tracesResponse = await langfuse.api.trace.list({
-      sessionId,
-      limit: 1,
-    });
-
-    const traces = tracesResponse.data || [];
-    const latestTrace = traces[0];
-
-    if (latestTrace) {
-      // Add a score to the existing trace to flag that user saved
-      await langfuse.api.ingestion.batch({
-        batch: [
-          {
-            type: 'score-create',
-            id: randomUUID(),
-            timestamp,
-            body: {
-              id: randomUUID(),
-              traceId: latestTrace.id,
-              name: 'diagram-saved',
-              value: 1,
-              comment: `User saved diagram as ${filename}.${format}`,
-            },
-          },
-        ],
-      });
-    }
-    // If no trace found, skip logging (user hasn't chatted yet)
-
-    return Response.json({ success: true, logged: !!latestTrace });
-  } catch (error) {
-    console.error('Langfuse save error:', error);
-    return Response.json({ success: false, error: 'Failed to log save' }, { status: 500 });
-  }
-}