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/search.py

@@ -36,9 +36,9 @@ from app.models import (
     Video,
     VideoState,
 )
-from app.routes.feed import _serialize, feed_columns
+from app.routes.feed import serialize, feed_columns
 from app.sync.subscriptions import apply_channel_details
-from app.sync.videos import _insert_stubs, apply_video_details, parse_dt
+from app.sync.videos import insert_stubs, apply_video_details, parse_dt
 from app.youtube import search_scrape
 from app.youtube.client import YouTubeClient, YouTubeError
 from app.youtube.shorts import make_client, probe_is_short
@@ -102,7 +102,7 @@ def _serialize_ids(db: Session, user: User, ids: list[str]) -> list[dict]:
         .outerjoin(state, and_(state.video_id == Video.id, state.user_id == user.id))
         .where(Video.id.in_(ids))
     ).all()
-    by_id = {r.id: _serialize(r) for r in rows}
+    by_id = {r.id: serialize(r) for r in rows}
     return [by_id[i] for i in ids if i in by_id]
 
 
@@ -136,7 +136,7 @@ def _ingest_candidates(
 
     # 2) Video stubs, flagged via_search. Existing rows keep their flags (ON CONFLICT DO
     #    NOTHING), so an organically-synced video stays organic.
-    _insert_stubs(
+    insert_stubs(
         db,
         [
             {