fix: BigInt literal compatibility and bot link code JSON parsing

- Replace 0n literals with BigInt(0) for ES2017 target compatibility
- Parse link code JSON to extract userId and check expiration (was
  passing raw JSON string as FK, causing constraint violation)

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

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

@@ -60,7 +60,7 @@ export async function uploadPackagePreview(
       data: {
         previewData: buffer,
         // Set previewMsgId to 0 as sentinel so hasPreview checks work
-        previewMsgId: 0n,
+        previewMsgId: BigInt(0),
       },
     });
 
@@ -125,7 +125,7 @@ export async function setPreviewFromExtract(
         previewData: extractReq.imageData,
         // Set previewMsgId to 0 as sentinel so hasPreview checks work
         // (original Telegram-matched previews have the actual message ID)
-        previewMsgId: 0n,
+        previewMsgId: BigInt(0),
       },
     });