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/sync/explore.py

@@ -23,7 +23,7 @@ from app.models import (
     Video,
     VideoState,
 )
-from app.sync.videos import _insert_stubs, _stub_from_playlist_item, apply_video_details
+from app.sync.videos import insert_stubs, stub_from_playlist_item, apply_video_details
 from app.utils import now_utc as _now
 from app.youtube.client import YouTubeClient
 
@@ -71,11 +71,11 @@ def explore_ingest_page(
     data = yt.get_playlist_items(channel.uploads_playlist_id, page_token)
     stubs: list[dict] = []
     for item in data.get("items", []):
-        stub = _stub_from_playlist_item(item, channel.id)
+        stub = stub_from_playlist_item(item, channel.id)
         if stub is not None:
             stub["via_explore"] = True
             stubs.append(stub)
-    inserted = _insert_stubs(db, stubs)
+    inserted = insert_stubs(db, stubs)
     self_enriched = _enrich_new(db, yt, [s["id"] for s in stubs])
     log.info(
         "explore ingest channel=%s ingested=%s enriched=%s", channel.id, inserted, self_enriched