refactor: R12 S3 — invert scheduler->routes, publish cross-module privates

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.

backend/app/routes/playlists.py

@@ -10,7 +10,7 @@ from app import quota
 from app.auth import current_user, has_read_scope, has_write_scope, require_write_scope
 from app.db import get_db
 from app.models import Channel, Playlist, PlaylistItem, User, Video, VideoState
-from app.routes.feed import _serialize, feed_columns
+from app.routes.feed import serialize, feed_columns
 from app.sync.playlists import (
     plan_push,
     push_playlist,
@@ -400,7 +400,7 @@ def get_playlist(
         "source": pl.source,
         "yt_playlist_id": pl.yt_playlist_id,
         "dirty": pl.dirty,
-        "items": [_serialize(r) for r in rows],
+        "items": [serialize(r) for r in rows],
     }