feat(grid_tasks): add kind column to split forecast vs analysis work

Prep for Stream A of Phase 3 rust migration: f00's native-duct merge,
NEXRAD, and commercial-link enrichment move to Rust via the same
grid_tasks work queue. kind='analysis' (single row per run) vs
kind='forecast' (18 rows per run) lets one Rust worker dispatch on
payload type without a second table.

Forecast default keeps existing Rust claim_next behaviour unchanged
during cutover.

priv/repo/migrations/20260419222624_add_kind_to_grid_tasks.exs

@@ -0,0 +1,24 @@
+defmodule Microwaveprop.Repo.Migrations.AddKindToGridTasks do
+  use Ecto.Migration
+
+  # `kind` splits the queue between forecast-hour work (the 18 f01..f18 rows
+  # Rust has owned since Phase 2) and analysis-hour work (f00, which Elixir
+  # still runs because it carries the native-level duct merge, NEXRAD, and
+  # commercial-link enrichment). Stream A of the Phase 3 migration plan
+  # ports those enrichments to Rust and flips f00 onto this queue too.
+  # Forecast rows default to 'forecast' so existing producers keep working
+  # during the transition.
+  def change do
+    alter table(:grid_tasks) do
+      add :kind, :string, null: false, default: "forecast"
+    end
+
+    create index(:grid_tasks, [:status, :kind])
+
+    # Replace (run_time, forecast_hour) uniqueness with a three-column key so
+    # an analysis row and a forecast row for f00 can coexist during the
+    # cutover. Post-cutover the analysis row replaces f00 outright.
+    drop_if_exists unique_index(:grid_tasks, [:run_time, :forecast_hour])
+    create unique_index(:grid_tasks, [:run_time, :forecast_hour, :kind])
+  end
+end