fix(issues): replace non-null assertions with null checks in checkout re-read

Two code paths in issueService.checkout() used rows[0]! when
re-reading an issue after stale-run adoption or self-ownership
verification. If the issue is deleted concurrently (company cascade,
API delete), rows[0] is undefined and withIssueLabels crashes with
an unhandled TypeError.

Replace both with rows[0] ?? null and throw notFound when the row
is missing, returning a clean 404 instead of an uncaught exception.

server/src/services/issues.ts

@@ -1469,7 +1469,8 @@ export function issueService(db: Db) {
           expectedCheckoutRunId: current.checkoutRunId,
         });
         if (adopted) {
-          const row = await db.select().from(issues).where(eq(issues.id, id)).then((rows) => rows[0]!);
+          const row = await db.select().from(issues).where(eq(issues.id, id)).then((rows) => rows[0] ?? null);
+          if (!row) throw notFound("Issue not found");
           const [enriched] = await withIssueLabels(db, [row]);
           return enriched;
         }
@@ -1481,7 +1482,8 @@ export function issueService(db: Db) {
         current.status === "in_progress" &&
         sameRunLock(current.checkoutRunId, checkoutRunId)
       ) {
-        const row = await db.select().from(issues).where(eq(issues.id, id)).then((rows) => rows[0]!);
+        const row = await db.select().from(issues).where(eq(issues.id, id)).then((rows) => rows[0] ?? null);
+        if (!row) throw notFound("Issue not found");
         const [enriched] = await withIssueLabels(db, [row]);
         return enriched;
       }