Move Era5MonthBatchWorker to its own queue

The slow CDS submit/poll/download cycle of the month-batch worker was
sharing the era5 queue with the cheap Era5FetchWorker router, and its
2 concurrent slots were permanently pinned by long-running batch jobs
while hundreds of router jobs starved. Give the batch worker its own
era5_batch queue (also 2 concurrent per pod) so the router has
dedicated capacity.

config/runtime.exs

@@ -141,6 +141,7 @@ if config_env() == :prod do
       terrain: 3,
       iemre: 3,
       era5: 2,
+      era5_batch: 2,
       rtma: 2,
       backfill_enqueue: 1
     ],

lib/microwaveprop/workers/era5_month_batch_worker.ex

@@ -13,9 +13,13 @@ defmodule Microwaveprop.Workers.Era5MonthBatchWorker do
   Uniqueness on the full arg set means Oban collapses duplicate enqueues
   for the same tile-month — multiple `Era5FetchWorker` requests for the
   same region collapse into a single batch.
+
+  Runs on its own `:era5_batch` queue so its slow CDS poll cycles don't
+  starve the fast `Era5FetchWorker` router that also lives in the ERA5
+  namespace.
   """
   use Oban.Worker,
-    queue: :era5,
+    queue: :era5_batch,
     max_attempts: 3,
     unique: [
       period: :infinity,