Implement MIP-01 #h query guardrails and ordering tests

priv/repo/migrations/20260313205654_add_event_tags_h_lookup_index.exs

@@ -0,0 +1,13 @@
+defmodule Parrhesia.Repo.Migrations.AddEventTagsHLookupIndex do
+  use Ecto.Migration
+
+  def up do
+    execute(
+      "CREATE INDEX event_tags_h_value_created_at_idx ON event_tags (value, event_created_at DESC) WHERE name = 'h'"
+    )
+  end
+
+  def down do
+    execute("DROP INDEX event_tags_h_value_created_at_idx")
+  end
+end