fix(search): typed text is a literal in every LIKE pattern

The feed's channel-name match and the live search's term fold built their
LIKE pattern from the typed text as it was, so `_` and `%` acted as
wildcards: `build_log` also matched `buildXlog`. The term fold also tested
a substring, so a term already present inside a longer word (`cat` in
`category`) was never folded into search_terms. It now compares whole
terms, escaped.

`like_escape` moves from the Plex query module to `app.utils`, beside
`to_tsquery_str`, and every pattern built from typed text goes through it.

backend/app/plex/query.py

@@ -298,10 +298,6 @@ class LibraryFilters:
         }
 
 
-def like_escape(s: str) -> str:
-    return s.replace("\\", "\\\\").replace("%", "\\%").replace("_", "\\_")
-
-
 # --- Watch-state scope (shared by the /library grid AND the /facets aggregation) ---------------
 #
 # The `show` scope (all | watched | in_progress | unwatched) filters the browse result by per-user