feat(notifications): mint long-lived JWT for native New Activity prefetch (Phase A)

Add BACKGROUND_JWT_EXPIRY_DAYS/SECONDS and accessTokenForBackgroundNotifications
via createEndorserJwtForDid; configureNativeFetcher uses it instead of getHeaders
so WorkManager prefetch is not stuck with a 60s access token. Interactive API
calls unchanged.

src/constants/backgroundJwt.ts

@@ -0,0 +1,9 @@
+/**
+ * JWT lifetime for native New Activity background prefetch (`configureNativeFetcher`).
+ * Phase A: single long-lived token minted in TS; see doc/plan-background-jwt-pool-and-expiry.md.
+ * Confirm max `exp` with Endorser before raising.
+ */
+export const BACKGROUND_JWT_EXPIRY_DAYS = 90;
+
+export const BACKGROUND_JWT_EXPIRY_SECONDS =
+  BACKGROUND_JWT_EXPIRY_DAYS * 24 * 60 * 60;