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>

bot/src/db/queries.ts

@@ -21,7 +21,16 @@ export async function findLinkByUserId(userId: string) {
 export async function validateLinkCode(code: string): Promise<string | null> {
   const key = `link_code:${code}`;
   const setting = await db.globalSetting.findUnique({ where: { key } });
-  return setting?.value ?? null;
+  if (!setting) return null;
+
+  try {
+    const parsed = JSON.parse(setting.value);
+    if (parsed.expiresAt && new Date(parsed.expiresAt) < new Date()) return null;
+    return parsed.userId ?? null;
+  } catch {
+    // Legacy format: value is the userId directly
+    return setting.value;
+  }
 }
 
 export async function deleteLinkCode(code: string): Promise<void> {