Layering cleanup, no behaviour change (ruff clean backend+scripts, pytest 193):
- Invert the scheduler->routes dependency: reset_demo_state (+ its demo-seed
helpers) move from routes/admin.py to a new service module app/demo.py.
Both routes/admin.py and scheduler.py now import it from app.demo, so the
scheduler no longer reaches into the routes layer — its lazy import (the
routes<->scheduler cycle workaround) is gone, hoisted to a top-level import.
- Publish the private symbols that were imported across module boundaries (the
underscore convention was lying): feed._serialize -> serialize,
feed._to_tsquery_str -> to_tsquery_str, videos._insert_stubs -> insert_stubs,
videos._stub_from_playlist_item -> stub_from_playlist_item,
worker._uploader_url -> uploader_url, plex._shared._item_or_404 ->
item_or_404. Scoped renames — the 5 other route modules keep their own truly
private _serialize helpers.
- _release_asset moves from routes/downloads.py to app/downloads/service.py as
the public release_asset (asset bookkeeping is a service concern); the routes
call it via service.release_asset.