perf(fleet): reduce IEM 429 storm and raise probe timeouts

Per-pod IEM rate limiter was 700ms (~1.4 req/sec); four pods put the
fleet at ~5.7 req/sec, above the observed 429 threshold. Retry storm
(Req exponential backoff up to 32s) piled concurrent sleeps on top of
live workers and tripped Bandit's acceptor + Oban's leader heartbeat.

- Raise default interval to 1500ms (~2.7 req/sec fleet).
- Hot pod /live timeout 3s → 5s (acceptor stalls under retry sleeps).
- prop-backfill exec probe period 30s → 120s, timeout 15s → 30s; the
  release CLI round-trips through distributed Erlang and 15s wasn't
  enough under load, restart-looping every ~10 min.

k8s/deployment-backfill.yaml

@@ -73,25 +73,27 @@ spec:
             - secretRef:
                 name: prop-secrets
           # No HTTP probes (no web server). Use exec probe against the
-          # running BEAM to confirm the VM is responsive. timeoutSeconds
-          # is generous because `rpc ':ok'` has to spawn a release CLI
-          # and round-trip through distributed Erlang; during an IEM
-          # 429 retry storm (WeatherFetchWorker sleeping 1-16 s between
-          # attempts) the scheduler is responsive but slow, and a 5 s
-          # probe timeout was tripping failureThreshold every ~10 min.
+          # running BEAM to confirm the VM is responsive. `rpc ':ok'`
+          # has to spawn a release CLI and round-trip through distributed
+          # Erlang; during an IEM 429 retry storm the scheduler is
+          # responsive but slow. 15 s still tripped failureThreshold
+          # every ~10 min, so the liveness probe now runs every 2 min
+          # with a 30 s ceiling — the BEAM itself would have crashed
+          # long before that if it were genuinely wedged, and rescue
+          # relies on the BEAM's own crash-loop handling anyway.
           startupProbe:
             exec:
               command: ["/bin/sh", "-c", "/app/bin/microwaveprop rpc ':ok'"]
             initialDelaySeconds: 10
-            periodSeconds: 5
-            failureThreshold: 24
-            timeoutSeconds: 15
+            periodSeconds: 10
+            failureThreshold: 18
+            timeoutSeconds: 30
           livenessProbe:
             exec:
               command: ["/bin/sh", "-c", "/app/bin/microwaveprop rpc ':ok'"]
-            periodSeconds: 30
+            periodSeconds: 120
             failureThreshold: 3
-            timeoutSeconds: 15
+            timeoutSeconds: 30
           resources:
             # 8GB T620 box — leave ~3GB for kubelet/containerd/OS.
             requests:

k8s/deployment.yaml

@@ -104,13 +104,17 @@ spec:
           # /health's `SELECT 1` to time out and the kubelet to SIGKILL
           # every replica at once. Readiness still points at /health so
           # DB outages drain the pod from Service endpoints gracefully.
+          # timeoutSeconds raised to 5s: under an IEM 429 retry storm
+          # Bandit's acceptor loop can stall briefly as the scheduler
+          # chews through retrying HTTP sleeps, and 3s was tripping
+          # failureThreshold across all 3 replicas at once.
           livenessProbe:
             httpGet:
               path: /live
               port: 5000
             periodSeconds: 10
             failureThreshold: 3
-            timeoutSeconds: 3
+            timeoutSeconds: 5
           resources:
             requests:
               cpu: 100m

lib/microwaveprop/weather/iem_rate_limiter.ex

@@ -12,14 +12,21 @@ defmodule Microwaveprop.Weather.IemRateLimiter do
   arrives.
 
   Configured via `config :microwaveprop, :iem_rate_limiter_interval_ms`
-  (defaults to 700ms ≈ 1.4 req/sec per pod; three pods give ~4 req/sec
-  cluster-wide, well under the empirically observed 429 threshold).
+  (defaults to 1500ms ≈ 0.67 req/sec per pod; four pods give ~2.7
+  req/sec cluster-wide). Previously set at 700ms, which put four pods
+  at ~5.7 req/sec fleet-wide — above IEM's observed 429 threshold —
+  and the resulting retry storm (Req exponential backoff up to 32 s)
+  piled concurrent HTTP sleeps on top of live workers until Bandit's
+  acceptor and Oban's leader heartbeat both started tripping. The
+  limiter is per-pod, so cluster-synchronous bursts are still possible
+  when every pod reserves `now + interval` simultaneously; moving to
+  a global limiter is out of scope for this tuning pass.
   In tests, pass `interval_ms: 0` to disable.
   """
 
   use GenServer
 
-  @default_interval_ms 700
+  @default_interval_ms 1500
 
   @spec start_link(keyword()) :: GenServer.on_start()
   def start_link(opts \\ []) do