feat(chat): per-request transcript budget so a long recording is not cut to its head (ADR-033)

chat.Request.MaxChars overrides the Service cap when positive. The 18,000-char
default (~22 min of speech) suits captions; an uploaded hour is ~50,000 chars
and truncation keeps only the head. This is reuse of the existing chat path,
not a fork.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/adapters/chat/chat.go

@@ -41,6 +41,11 @@ type Request struct {
 	Transcript string
 	History    []Turn
 	Question   string
+	// MaxChars, when positive, overrides the Service's transcript budget for this
+	// request. The Service default is sized for YouTube captions (~22 minutes of
+	// speech); an uploaded recording is much longer, and truncation keeps only the
+	// head (ADR-033). Zero or negative means "use the Service default".
+	MaxChars int
 }
 
 // Reply is the model's answer plus whether the transcript was bounded to fit the
@@ -113,7 +118,11 @@ func (s *Service) Answer(ctx context.Context, req Request) (Reply, error) {
 		model = s.DefaultModel("")
 	}
 
-	transcript, truncated := truncate(req.Transcript, s.maxChars)
+	budget := s.maxChars
+	if req.MaxChars > 0 {
+		budget = req.MaxChars
+	}
+	transcript, truncated := truncate(req.Transcript, budget)
 	system := buildSystem(transcript, truncated)
 	user := buildUser(req.History, req.Question)