Move import staging files/groups/hints/suggestions controller logic out of web_server.py and into core.imports.routes behind an ImportRouteRuntime dependency object. Keep the existing Flask routes as thin compatibility wrappers so the UI endpoint surface stays unchanged.
Add focused tests for staging file filtering, album grouping, hint generation, cached suggestions, empty missing staging paths, and error payloads from failed path/metadata reads.
Verification: py_compile passed for web_server.py, core/imports/routes.py, and tests/imports/test_import_routes.py. A bundled-Python smoke pass covered the extracted helper behavior; pytest was not available in this Windows shell because the bundled Python lacks pytest and the repo venv is WSL/Linux-only here.
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