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/imapx/copy.go

@@ -23,6 +23,10 @@ type CopyDeps struct {
 	// (potentially long) envelope fetch, with the message count in the source
 	// folder — for progress visibility.
 	OnFolder func(srcFolder, dstFolder string, total int64)
+	// OnScan is called during the streaming metadata pass with how many of the
+	// folder's messages have been examined so far — so the UI shows movement
+	// while dedup decisions are made, before bodies start copying.
+	OnScan func(scanned, total int64)
 }
 
 // CopyResult summarizes the outcome of one CopyFolder run.
@@ -46,48 +50,77 @@ func CopyFolder(ctx context.Context, src, dst *imapclient.Client, srcFolder, dst
 	if err != nil {
 		return res, fmt.Errorf("examine src %q: %w", srcFolder, err)
 	}
+	total := int64(sel.NumMessages)
 	if deps.OnFolder != nil {
-		deps.OnFolder(srcFolder, dstFolder, int64(sel.NumMessages))
+		deps.OnFolder(srcFolder, dstFolder, total)
 	}
-	if sel.NumMessages == 0 {
+	if total == 0 {
 		return res, nil
 	}
 
-	// 1) Collect envelope+uid+size for every message (cheap pass, no bodies).
-	metaSet := imap.SeqSet{imap.SeqRange{Start: 1, Stop: sel.NumMessages}}
-	metas, err := src.Fetch(metaSet, &imap.FetchOptions{
-		UID: true, Envelope: true, RFC822Size: true, Flags: true, InternalDate: true,
-	}).Collect()
-	if err != nil {
-		return res, fmt.Errorf("fetch meta: %w", err)
-	}
-
 	// dst folder must exist (idempotent create; ignore "already exists").
 	_ = dst.Create(dstFolder, nil).Wait()
 
-	for _, m := range metas {
+	// Pass 1: STREAM metadata (no bodies) via Next(), dedup as we go, and queue
+	// only the new messages. Streaming (not Collect) means progress shows during
+	// the scan and memory stays flat — we hold small meta for new messages only.
+	type queued struct {
+		uid          imap.UID
+		key          string
+		flags        []imap.Flag
+		internalDate time.Time
+	}
+	var todo []queued
+	metaSet := imap.SeqSet{imap.SeqRange{Start: 1, Stop: sel.NumMessages}}
+	fc := src.Fetch(metaSet, &imap.FetchOptions{
+		UID: true, Envelope: true, RFC822Size: true, Flags: true, InternalDate: true,
+	})
+	var scanned int64
+	for {
 		if err := ctx.Err(); err != nil {
+			_ = fc.Close()
 			return res, err
 		}
-
-		key := MessageKey(m.Envelope, m.RFC822Size)
-		already, err := deps.IsMigrated(key)
+		msg := fc.Next()
+		if msg == nil {
+			break
+		}
+		buf, err := msg.Collect()
 		if err != nil {
 			res.Errors++
 			continue
 		}
-		if already {
+		scanned++
+		key := MessageKey(buf.Envelope, buf.RFC822Size)
+		already, err := deps.IsMigrated(key)
+		if err != nil {
+			res.Errors++
+		} else if already {
 			res.Skipped++
 			if deps.OnProgress != nil {
 				deps.OnProgress(res.Copied, res.Skipped)
 			}
-			continue
+		} else {
+			todo = append(todo, queued{uid: buf.UID, key: key, flags: buf.Flags, internalDate: buf.InternalDate})
 		}
-		if err := streamOne(src, dst, dstFolder, m.UID, m.Flags, m.InternalDate); err != nil {
+		if deps.OnScan != nil {
+			deps.OnScan(scanned, total)
+		}
+	}
+	if err := fc.Close(); err != nil {
+		return res, fmt.Errorf("fetch meta %q: %w", srcFolder, err)
+	}
+
+	// Pass 2: fetch bodies for the queued (new) messages, one at a time.
+	for _, q := range todo {
+		if err := ctx.Err(); err != nil {
+			return res, err
+		}
+		if err := streamOne(src, dst, dstFolder, q.uid, q.flags, q.internalDate); err != nil {
 			res.Errors++
 			continue
 		}
-		if err := deps.MarkMigrated(dstFolder, key); err != nil {
+		if err := deps.MarkMigrated(dstFolder, q.key); err != nil {
 			res.Errors++
 			continue
 		}