feat(web): chat over an upload opens with App.UploadChatModel

The Chatter's own default is the summarizer's (koala/phi4-mini, 8,192-token
context), which answers HTTP 400 on an hour of speech. Empty keeps the old
behaviour.

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

internal/web/handlers.go

@@ -117,6 +117,11 @@ type App struct {
 	// recording. The caption default (18,000 chars, ~22 minutes of speech) would
 	// keep only the first third of an hour-long meeting.
 	UploadChatMaxChars int
+	// UploadChatModel is the model chat over an uploaded recording opens with. Empty
+	// means the Chatter's own default. It exists because that default is the
+	// summarizer's (koala/phi4-mini, 8,192-token context) and cannot hold an hour of
+	// speech.
+	UploadChatModel string
 	// Processing tracks in-flight immediate summarizations so the status endpoint
 	// shows the animation until the summary lands. The zero value is ready to use.
 	Processing ProcessingSet