Prevent duplicate PropagationGridWorker jobs via Oban unique constraint

Worker now uses unique: [period: 3600, states: [:available, :scheduled,
:executing, :retryable]] so Oban atomically prevents duplicates.
FreshnessMonitor simplified — no longer does manual oban_jobs query.

lib/microwaveprop/propagation/freshness_monitor.ex

@@ -53,20 +53,9 @@ defmodule Microwaveprop.Propagation.FreshnessMonitor do
   end
 
   defp enqueue_if_not_queued do
-    # Only enqueue if there isn't already a pending/running grid worker job
-    import Ecto.Query
-
-    pending =
-      Microwaveprop.Repo.exists?(
-        from(j in "oban_jobs",
-          where:
-            j.worker == "Microwaveprop.Workers.PropagationGridWorker" and
-              j.state in ["available", "scheduled", "executing"]
-        )
-      )
-
-    if !pending do
-      Oban.insert(PropagationGridWorker.new(%{}))
-    end
+    # Oban unique constraint on PropagationGridWorker prevents duplicates —
+    # if a job is already available/scheduled/executing/retryable within the
+    # last hour, insert is a no-op.
+    Oban.insert(PropagationGridWorker.new(%{}))
   end
 end

lib/microwaveprop/workers/propagation_grid_worker.ex

@@ -7,7 +7,8 @@ defmodule Microwaveprop.Workers.PropagationGridWorker do
 
   use Oban.Worker,
     queue: :propagation,
-    max_attempts: 3
+    max_attempts: 3,
+    unique: [period: 3600, states: [:available, :scheduled, :executing, :retryable]]
 
   alias Microwaveprop.Propagation
   alias Microwaveprop.Propagation.Grid