Applies the three findings from the /code-review high pass on this stack:
1. to_tsquery_str moved from app.feedquery to app.utils — it's a generic
tsquery sanitizer with no feed dependency; housing it in the feed
read-model coupled the Plex browse module to the feed domain. Both feed
and plex now import it from the neutral util module.
2. feedquery.decode_cursor no longer raises fastapi.HTTPException — it raises
a plain ValueError, and routes/feed.get_feed translates it to the HTTP 400.
The read-model no longer imports fastapi, matching its "no request state"
docstring.
3. Dropped the leading underscore on the helpers that are now a cross-module
public API: feedquery {filtered_query, sort_keys, order_by, keyset_predicate,
encode_cursor, decode_cursor}; plex.query {TS_CONFIG, FIELD_COLUMNS,
field_recheck, split_csv, added_cutoff, meta_filter_conds, apply_meta_filters,
like_escape}; plex.cards {movie_card, show_card, episode_card, leaf_card}.
Genuinely-internal helpers (_channel_url, _saved_expr, _ADDED_WINDOWS,
_lib_key) keep their underscore.
Pure code motion + rename. DB-lane suite 287 passed / 2 skipped; ruff clean.