feat: group merge, ZIP/reply/caption grouping, integrity audit

Group merge UI:
- Add mergeGroups query and mergeGroupsAction server action
- Add "Start Merge" / "Merge Here" buttons to group row actions
- Two-step UX: click Start on source, click Merge Here on target

ZIP path prefix grouping (Signal 7):
- Compare PackageFile.path root folders across ungrouped packages
- Auto-group if 2+ packages share the same dominant root folder

Reply chain grouping (Signal 6):
- Capture reply_to_message_id during channel scanning
- Group archives that reply to the same root message
- Add replyToMessageId field to Package schema

Caption fuzzy match grouping (Signal 8):
- Capture source caption during channel scanning
- Normalize captions (strip extensions, extract significant words)
- Group packages with matching normalized caption keys
- Add sourceCaption field to Package schema

Periodic integrity audit:
- Check multipart packages for completeness (parts vs destMessageIds)
- Detect orphaned indexes (destChannelId set but no destMessageId)
- Runs after each ingestion cycle, deduplicates notifications

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

worker/src/tdlib/download.ts

@@ -39,6 +39,7 @@ interface TdMessage {
   id: number;
   date: number;
   media_album_id?: string;
+  reply_to_message_id?: number;
   content: {
     _: string;
     document?: {
@@ -216,6 +217,8 @@ export async function getChannelMessages(
             fileSize: BigInt(doc.document.size),
             date: new Date(msg.date * 1000),
             mediaAlbumId: msg.media_album_id && msg.media_album_id !== "0" ? msg.media_album_id : undefined,
+            replyToMessageId: msg.reply_to_message_id ? BigInt(msg.reply_to_message_id) : undefined,
+            caption: msg.content?.caption?.text || undefined,
           });
           continue;
         }