Check DRE vault upkeep and perform when needed (cron)

Expected schedule: every 10 minutes.

For each of DRE_VAULT1, DRE_VAULT2, and DRE_WITHDRAWAL_KEEPER_BOT (see src/config/addresses.ts), calls on-chain checkUpkeep('0x') via the configured RPC endpoints. When upkeepNeeded is true, signs and submits performUpkeep(performData) using UPKEEP_PRIVATE_KEY; vaults that do not need upkeep are skipped. Returns 204 with an empty body on success. Requires X-CRON-API-KEY matching CRON_API_KEY.