feat(shared): Add oiat field to JwtHeader type

Session Minter uses oiat (original_issued_at) in the JWT header to
track when token claims were last assembled from the DB. Edge
re-mints copy this value forward, so consumers can determine claim
freshness regardless of how many times the token was re-signed.

Marked @internal so developers don't depend on this field.

Author: nikosdouvlis

.changeset/session-minter-oiat-type.md

@@ -0,0 +1,5 @@
+---
+'@clerk/shared': patch
+---
+
+Add `oiat` (original_issued_at) field to `JwtHeader` type for Session Minter monotonic token freshness checks.

packages/shared/src/types/jwtv2.ts

@@ -25,6 +25,8 @@ export interface JwtHeader {
   'x5t#S256'?: string;
   x5t?: string;
   x5c?: string | string[];
+  /** @internal - used by Session Minter for monotonic token freshness checks. Do not depend on this field. */
+  oiat?: number;
 }
 
 declare global {