Make diarization chunk length configurable (Auto + presets)

Chunk size was hardcoded at 2.5-min bodies. Add a Settings control:
Auto / Standard 2.5min / Large group 60s / Fine 90s. Shorter chunks keep fewer
simultaneous speakers per window (Sortformer resolves ~4/chunk), useful for large
calls, at some cost to speed and cross-chunk voice matching.

- ChunkMode (new, pure/testable): mode → body seconds; Auto picks 60s when >4
  participants were detected, else 150s; overlap + single-chunk threshold scale
  with the body length.
- AppSettings.chunkMode (+ typed `chunk`); SettingsView picker with explanation.
- TranscriptPipeline.process gains chunkSeconds; derives overlap/threshold from it.
- SessionController resolves the body from the setting + the session's detected
  participant count (visual_timeline participants) for both send + re-process.
- Participant roster now counts EVERY tile OCR'd, not just who spoke
  (TimelineBuilder.observedNames → VisualObserver → VisualCapture), so the Auto
  call-size signal is meaningful even though speaking-detection is sparse.

Tests: ChunkMode resolution, overlap scaling, short-body re-chunking. 69 pass.

Ten31Transcripts/UI/SettingsView.swift

@@ -39,6 +39,12 @@ struct SettingsView: View {
             Section("Transcription") {
                 Toggle("Auto-send recordings to backend", isOn: $settings.autoSendOnStop)
                 Toggle("Reconcile speakers (merge splits + name from content)", isOn: $settings.reconcileSpeakers)
+                Picker("Chunk length", selection: $settings.chunkMode) {
+                    ForEach(ChunkMode.allCases) { Text($0.label).tag($0.rawValue) }
+                }
+                Text("How finely audio is split for diarization. Shorter chunks keep fewer simultaneous speakers per window (the diarizer resolves ~4 at a time), at some cost to speed and voice matching. Auto uses 60-sec chunks when more than \(ChunkMode.autoLargeThreshold) people are detected on the call, else 2.5 min.")
+                    .font(.caption)
+                    .foregroundStyle(.secondary)
                 Toggle("Build readable recap (topics + highlights)", isOn: $settings.recapEnabled)
                 HStack {
                     Picker("Default recap template", selection: $settings.defaultTemplateId) {