debug: add entry-point log to sendMessage handler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/routers.ts

@@ -258,6 +258,7 @@ export const appRouter = router({
         content: z.string().min(1).max(2000),
       }))
       .mutation(async ({ input, ctx }) => {
+        console.log(`[sendMessage] sessionId=${input.sessionId} userId=${ctx.user?.id ?? "anon"} content="${input.content.slice(0, 60)}"`);
         const conversation = await getConversationBySessionId(input.sessionId);
         if (!conversation) throw new Error("Conversation not found");