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>

prisma/schema.prisma

@@ -474,6 +474,8 @@ model Package {
   partCount       Int         @default(1)
   fileCount       Int         @default(0)
   tags            String[]    @default([])
+  sourceCaption   String?     // Caption text from source Telegram message
+  replyToMessageId BigInt?    // reply_to_message_id from source message (for reply chain grouping)
   previewData     Bytes?      // JPEG thumbnail from nearby Telegram photo (stored as raw bytes)
   previewMsgId    BigInt?     // Telegram message ID of the matched photo
   packageGroupId  String?