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/feedquery.py

@@ -30,7 +30,7 @@ from app.models import (
     Video,
     VideoState,
 )
-from app.utils import iso, to_tsquery_str
+from app.utils import iso, like_escape, to_tsquery_str
 
 
 def _channel_url(channel_id: str, handle: str | None) -> str:
@@ -272,8 +272,9 @@ def filtered_query(
         # `unaccent(channels.title) ILIKE '%q%'` straight into the per-video predicate defeated the
         # search_vector GIN — the planner detoasted every non-matching tsvector to test the `@@`
         # (~2.6s/search on the prod-sized set); channel_id.in_(...) keeps the GIN as a BitmapOr branch
-        # (PERF-12). Empty match -> false() so the OR collapses to the text match alone.
-        like = func.unaccent(f"%{q}%")
+        # (PERF-12). Empty match -> false() so the OR collapses to the text match alone. The query
+        # is a literal inside the pattern: `_` in `my_channel` is not a wildcard.
+        like = func.unaccent(f"%{like_escape(q)}%")
         matching_channel_ids = (
             db.execute(select(Channel.id).where(func.unaccent(Channel.title).ilike(like)))
             .scalars()