feat(notifications): mint JWT pool for native fetcher; log API response

- Mint BACKGROUND_JWT_POOL_SIZE (90 + 10) distinct background JWTs with
  unique jti; pass jwtTokens from nativeFetcherConfig into configureNativeFetcher.
- Android TimeSafariNativeFetcher: overload configure with jwtTokenPool;
  select bearer via epoch day mod pool size; fall back to primary jwtToken.
- Log truncated plansLastUpdatedBetween response at DEBUG for prefetch debugging.

src/constants/backgroundJwt.ts

@@ -1,9 +1,15 @@
 /**
  * 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.
+ * 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;
+
+/** Headroom for retries / tests; pool size should be ≥ expiryDays + buffer. */
+export const BACKGROUND_JWT_POOL_BUFFER = 10;
+
+/** Distinct JWT strings minted per configure (duplicate-JWT / daily slot). */
+export const BACKGROUND_JWT_POOL_SIZE =
+  BACKGROUND_JWT_EXPIRY_DAYS + BACKGROUND_JWT_POOL_BUFFER;