feat: stream the metadata scan instead of Collect

CopyFolder now streams envelope metadata via Next() in a first pass (dedup +
queue new messages), then streams bodies for new ones in a second pass —
no more blocking Collect of the whole folder with zero feedback, and memory
stays flat (only new-message meta is held).

- imapx: two-pass streaming CopyFolder + CopyDeps.OnScan(scanned,total)
- orchestrator: throttled 'scan' events during the metadata pass
- web: per-account 'scanning folder: X/N' line under the progress bar;
  scan events kept out of the log to avoid flooding

Verified on greenmail: idempotency and internal-date preservation still hold.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

internal/orchestrator/orchestrator.go

@@ -301,7 +301,7 @@ func (o *Orchestrator) runAccount(ctx context.Context, task store.Task, runID in
 	var baseCopied, baseSkipped int64
 	var curFolder string
 	var curTotal int64
-	var lastEmit time.Time
+	var lastEmit, lastScanEmit time.Time
 	deps := imapx.CopyDeps{
 		IsMigrated:   func(k string) (bool, error) { return o.store.IsMigrated(ctx, a.ID, k) },
 		MarkMigrated: func(folder, k string) error { return o.store.MarkMigrated(ctx, a.ID, folder, k) },
@@ -331,6 +331,18 @@ func (o *Orchestrator) runAccount(ctx context.Context, task store.Task, runID in
 				"folder": srcFolder, "dst_folder": dstFolder, "messages": total,
 			}})
 		},
+		// Fires while streaming metadata (dedup scan) so the UI shows movement
+		// before bodies start copying. Throttled to ~4/sec, always emit the last.
+		OnScan: func(scanned, total int64) {
+			now := time.Now()
+			if now.Sub(lastScanEmit) < 250*time.Millisecond && scanned < total {
+				return
+			}
+			lastScanEmit = now
+			o.hub.Publish(wshub.Event{Type: "scan", TaskID: task.ID, Data: map[string]any{
+				"account_id": a.ID, "folder": curFolder, "scanned": scanned, "folder_total": total,
+			}})
+		},
 	}
 	for _, fp := range plan {
 		if actx.Err() != nil {