Update Dockerfiles and compose paths for new flat structure

backend/Dockerfile

@@ -3,10 +3,10 @@ WORKDIR /app
 
 # Copy everything and restore as distinct layers
 COPY . .
-RUN dotnet restore apps/server-net/Knot.sln
+RUN dotnet restore Knot.sln
 
 # Build and publish a release
-RUN dotnet publish apps/server-net/src/Host/Host.csproj -c Release -o out
+RUN dotnet publish src/Host/Host.csproj -c Release -o out
 
 # Build runtime image
 FROM mcr.microsoft.com/dotnet/aspnet:10.0-preview