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/plex/_shared.py

@@ -12,7 +12,7 @@ from app.models import (
 log = logging.getLogger("siftlode.plex")
 
 
-def _item_or_404(db: Session, rating_key: str) -> PlexItem:
+def item_or_404(db: Session, rating_key: str) -> PlexItem:
     it = db.query(PlexItem).filter_by(rating_key=str(rating_key)).first()
     if it is None:
         raise HTTPException(status_code=404, detail="Unknown Plex item")

backend/app/routes/plex/browse.py

@@ -32,8 +32,8 @@ from app.plex import sync as plex_sync
 from app.plex.client import PlexClient, PlexError, PlexNotConfigured
 from app.routes._common import owned_or_404
 from app.routes.admin import admin_user
-from app.routes.feed import _to_tsquery_str
-from app.routes.plex._shared import _item_or_404
+from app.routes.feed import to_tsquery_str
+from app.routes.plex._shared import item_or_404
 
 router = APIRouter(prefix="/api/plex", tags=["plex"])
 
@@ -258,7 +258,7 @@ def unified_library(
     want_movies = scope in ("movie", "both") and bool(movie_lib_ids)
     want_shows = scope in ("show", "both") and bool(show_lib_ids)
 
-    ts = _to_tsquery_str(q) if q else None
+    ts = to_tsquery_str(q) if q else None
     tsq = func.to_tsquery(_TS_CONFIG, ts) if ts else None
 
     selects = []
@@ -1561,7 +1561,7 @@ def item_detail(
 ) -> dict:
     """Everything the player needs: metadata, cast, intro/credit markers (from Plex), episode
     prev/next, and the per-user resume position + watch status."""
-    it = _item_or_404(db, rating_key)
+    it = item_or_404(db, rating_key)
     st = db.query(PlexState).filter_by(user_id=user.id, item_id=it.id).first()
 
     rich: dict = {}

backend/app/routes/plex/playback.py

@@ -33,7 +33,7 @@ from app.plex import people as plex_people
 from app.plex import stream as plex_stream
 from app.plex import watch_sync as plex_watch
 from app.plex.client import PlexClient, PlexError, PlexNotConfigured
-from app.routes.plex._shared import _item_or_404
+from app.routes.plex._shared import item_or_404
 
 router = APIRouter(prefix="/api/plex", tags=["plex"])
 
@@ -105,7 +105,7 @@ def subtitle(
         return Response(subtitles.shift_vtt(hit, offset), media_type="text/vtt")
     try:
         with SessionLocal() as db:
-            it = _item_or_404(db, rating_key)
+            it = item_or_404(db, rating_key)
             with PlexClient(db) as plex:
                 part = ((plex.metadata(it.rating_key) or {}).get("Media") or [{}])[
                     0
@@ -236,7 +236,7 @@ def item_progress(
     `final` (sent on the pause/pagehide/unmount flush, not the periodic 10s checkpoint) is what
     debounces the Plex resume push: we mirror the settled position to Plex only on a final flush, so
     a single watch doesn't spray `/:/timeline` calls at the server every 10 seconds."""
-    it = _item_or_404(db, rating_key)
+    it = item_or_404(db, rating_key)
     position = max(0, int(body.position_seconds))
     duration = int(body.duration_seconds)
     final = body.final
@@ -307,7 +307,7 @@ def item_state(
     """Set the per-user watch status (new|watched|hidden). watched→scrobble / →new→unscrobble push
     to a linked Plex account immediately; `hidden` is a Siftlode-only concept with no Plex analogue,
     so it never touches Plex."""
-    it = _item_or_404(db, rating_key)
+    it = item_or_404(db, rating_key)
     status = body.status
     if status not in ("new", "watched", "hidden"):
         raise HTTPException(status_code=400, detail="Invalid status")
@@ -434,7 +434,7 @@ def stream_session(
     non-default audio track (`audio` = stream ordinal) forces the HLS path so the chosen audio can be
     muxed in. Subtitles are independent WebVTT tracks (GET /subtitle) — no session restart."""
     plex_stream.reap_idle()  # opportunistic cleanup of idle sessions on each new playback
-    it = _item_or_404(db, rating_key)
+    it = item_or_404(db, rating_key)
     if it.playable == "transcode":
         raise HTTPException(
             status_code=501, detail="This format needs transcoding (a later phase)."
@@ -471,7 +471,7 @@ def stream_file(
     """Serve the raw local media file with HTTP range support. Used both for direct-play (inline)
     and for the player's "download original" button (`?download=1` → attachment). The download is
     the untouched physical file — no re-encode/repackage."""
-    it = _item_or_404(db, rating_key)
+    it = item_or_404(db, rating_key)
     src = plex_paths.local_media_path(db, it.file_path)
     if src is None:
         raise HTTPException(status_code=404, detail="Media file not found on disk")
@@ -512,7 +512,7 @@ def stream_hls_file(
     if s is None:
         # No session yet (e.g. reaped): the ENTRY playlist may auto-start one at 0 — master.m3u8 for a
         # multi-audio item, index.m3u8 for a single-audio remux item.
-        it = _item_or_404(db, rating_key)
+        it = item_or_404(db, rating_key)
         if filename == "master.m3u8":
             s = plex_stream.start_session(db, it, 0.0, multi=True)
         elif filename == "index.m3u8" and it.playable == "remux":