fix: add race condition guard and null check in group queries

- createOrFindPackageGroup: catch unique constraint violation from
  concurrent creates and fall back to findFirst
- createManualGroup: guard against empty package results before
  accessing first element

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

src/lib/telegram/queries.ts

@@ -609,6 +609,9 @@ export async function createManualGroup(name: string, packageIds: string[]) {
     where: { id: { in: packageIds } },
     select: { sourceChannelId: true },
   });
+  if (pkgs.length === 0) {
+    throw new Error("No matching packages found");
+  }
   const channelIds = new Set(pkgs.map((p) => p.sourceChannelId));
   if (channelIds.size > 1) {
     throw new Error("Cannot group packages from different channels");