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/worker.py

@@ -262,7 +262,7 @@ def _finish_from_cache(job_id: int, asset_id: int) -> None:
         db.commit()
 
 
-def _uploader_url(info: dict) -> str | None:
+def uploader_url(info: dict) -> str | None:
     """The channel/uploader page URL for the auto-clickable channel line. YouTube gives a
     `channel_url`/`uploader_url` directly; Facebook gives neither but exposes a numeric
     `uploader_id` (the page id) that resolves at facebook.com/<id>. Returns None for sources with
@@ -290,7 +290,7 @@ def _fill_asset_meta_early(asset_id: int, info: dict) -> None:
         # the final title with its own source_ref fallback.
         asset.title = normalize_title(info.get("title")) or info.get("id")
         asset.uploader = info.get("uploader") or info.get("channel")
-        asset.uploader_url = _uploader_url(info)
+        asset.uploader_url = uploader_url(info)
         asset.thumbnail_url = info.get("thumbnail")
         asset.duration_s = int(info["duration"]) if info.get("duration") else None
         asset.upload_date = _parse_upload_date(info.get("upload_date"))
@@ -638,7 +638,7 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
                 asset.acodec = info.get("acodec") if info.get("acodec") not in (None, "none") else None
                 asset.title = meta.title
                 asset.uploader = meta.uploader
-                asset.uploader_url = _uploader_url(info)
+                asset.uploader_url = uploader_url(info)
                 asset.upload_date = meta.upload_date
                 asset.thumbnail_url = meta.thumbnail_url
                 # Record a self-hosted poster for every download: it's the reliable link-preview