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/Session/SessionController.swift

@@ -378,12 +378,15 @@ final class SessionController: ObservableObject {
         let settings = self.settings
         let pipeline = TranscriptPipeline(baseURL: settings.backendBaseURL,
                                           skipTLS: settings.skipTLSVerification, voiceprints: voiceprints)
+        // Resolve the diarization chunk length from the setting; "Auto" uses the
+        // participant count the visual capture saw for this session.
+        let chunkSeconds = settings.chunk.bodySeconds(participantCount: Self.participantCount(in: inputs.folder))
         do {
             let speakers = try await pipeline.process(
                 sessionFolder: inputs.folder, sessionId: inputs.sessionId, app: inputs.app,
                 micURL: inputs.micURL, systemURL: inputs.systemURL, mixedURL: inputs.mixedURL,
                 timeline: inputs.timeline, selfSpans: inputs.selfSpans, selfName: inputs.selfName,
-                systemHealthy: inputs.systemHealthy,
+                systemHealthy: inputs.systemHealthy, chunkSeconds: chunkSeconds,
                 progress: { done, total in await MainActor.run { self.transcriptStatus = .processing(done, total) } })
             self.transcriptStatus = .done(speakers: speakers.speakers.count, segments: speakers.segments.count)
             try Task.checkCancellation()
@@ -531,6 +534,16 @@ final class SessionController: ObservableObject {
         }
     }
 
+    /// Detected participant count from a session's visual timeline, for "Auto" chunk
+    /// sizing. Nil when there's no visual timeline (audio-only) so callers keep the
+    /// default body length. Counts everyone OCR'd on the call, not just who spoke.
+    private static func participantCount(in folder: URL) -> Int? {
+        guard let data = try? Data(contentsOf: folder.appendingPathComponent("visual_timeline.json")),
+              let vt = try? JSONDecoder().decode(VisualTimeline.self, from: data),
+              !vt.participants.isEmpty else { return nil }
+        return vt.participants.count
+    }
+
     /// The remote (vision) visual-timeline segments saved for a session, if any.
     private static func remoteTimeline(in folder: URL) -> [VisualTimeline.Segment] {
         guard let data = try? Data(contentsOf: folder.appendingPathComponent("visual_timeline.json")),