refactor(mcp): flatten to mcp_server package, drop src layout

Rename the import package surfsense_mcp -> mcp_server and remove the
src/ layer so the project mirrors the backend's shape (project folder
!= package name, e.g. surfsense_backend/app). Kills the redundant
surfsense_mcp/src/surfsense_mcp nesting.

Distribution name and console command (surfsense-mcp) are unchanged;
only python -m and internal import paths move to mcp_server. Dockerfile
CMD updated; no PYTHONPATH added since the editable install already
makes the package importable.

surfsense_mcp/Dockerfile

@@ -33,4 +33,4 @@ ENV PYTHONUNBUFFERED=1 \
 
 EXPOSE 8080
 
-CMD ["python", "-m", "surfsense_mcp"]
+CMD ["python", "-m", "mcp_server"]