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>

src/app/(app)/stls/actions.ts

@@ -10,6 +10,7 @@ import {
   createManualGroup,
   removePackageFromGroup,
   dissolveGroup,
+  mergeGroups,
 } from "@/lib/telegram/queries";
 
 const ALLOWED_IMAGE_TYPES = [
@@ -435,6 +436,26 @@ export async function updateGroupPreviewAction(
   }
 }
 
+export async function mergeGroupsAction(
+  targetGroupId: string,
+  sourceGroupId: string
+): Promise<ActionResult> {
+  const session = await auth();
+  if (!session?.user?.id) return { success: false, error: "Unauthorized" };
+
+  if (targetGroupId === sourceGroupId) {
+    return { success: false, error: "Cannot merge a group with itself" };
+  }
+
+  try {
+    await mergeGroups(targetGroupId, sourceGroupId);
+    revalidatePath("/stls");
+    return { success: true, data: undefined };
+  } catch {
+    return { success: false, error: "Failed to merge groups" };
+  }
+}
+
 export async function sendAllInGroupAction(
   groupId: string
 ): Promise<ActionResult> {