the ZIM fallback fires fast, and reads Russian in Russian (V-508)

Verification, as the task asked. Drove что такое фотосинтез through
/api/chat with the search reachable, with the container stopped, and with
the host blackholed. Kiwix claims the turn in both failure cases, and a
stopped container costs nothing: DNS fails and the ZIM answers inside the
same second.

The blackhole is the case that hurts. The search waited its full 8-second
budget before the ZIM was asked and the turn took 15.4s against 3.5, which
he sits through with nothing being said. So the connect phase alone is now
capped at 1.5s. A reachable instance that is merely slow keeps the whole
budget, because it is fanning out to real engines.

The RU Wikipedia ZIM is on the box (owner moved it into the kiwix zims
dir), and kiwix-serve picked it up. A Cyrillic question now searches
book_ru verbatim and skips the RU->EN rewrite: that rewriter is the
workaround for an English book, and against a Russian one it is a
translation of his own words back at him. Catalog names come from the
filename, not the <name> field — books.name=wikipedia_ru_all returns
nothing.

Measurement in docs/evals/2026-08-05-kiwix-offline-fallback.md. The RU book
answering a driven turn needs a rebuild and is not verified yet.

internal/config/config.go

@@ -1085,6 +1085,17 @@ type KiwixConfig struct {
 	// query at a time.
 	Book string `json:"book,omitempty"`
 
+	// BookRU — the ZIM to search when the question is in Russian, by the same
+	// catalog name. Empty ⇒ every question goes to Book.
+	//
+	// It exists because the rewriter is a workaround, not a feature (V-508). An
+	// English ZIM cannot match a Russian sentence, so the resident model turns
+	// the question into English keywords first, and that costs a model call and
+	// loses whatever the keywords drop. A Russian ZIM matches the question as he
+	// asked it. So a Cyrillic question searches this book verbatim and skips the
+	// rewrite, and the English book keeps answering English ones.
+	BookRU string `json:"book_ru,omitempty"`
+
 	// MaxResults — how many hits are asked for. 0 ⇒ DefaultKiwixResults.
 	// Only the top few reach the phraser regardless; the rest are context the
 	// snippet ranking throws away.