Fix duplicate indexes and missing socket assigns

- Remove duplicate received_at indexes (packets_received_at_asc_idx and packets_recent_hour_idx)
  that were consuming 423 MB, keeping only packets_received_at_idx
- Fix KeyError by initializing missing :all_packets and :visible_packets in socket assigns
- All tests now pass successfully

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/aprsme_web/live/map_live/index.ex

@@ -153,6 +153,8 @@ defmodule AprsmeWeb.MapLive.Index do
   defp assign_defaults(socket, one_hour_ago) do
     assign(socket,
       packets: [],
+      all_packets: %{},
+      visible_packets: %{},
       page_title: "APRS Map",
       packet_state: PacketManager.init_packet_state(),
       station_popup_open: false,

priv/repo/migrations/20250715155632_remove_duplicate_received_at_indexes.exs

@@ -0,0 +1,23 @@
+defmodule Aprsme.Repo.Migrations.RemoveDuplicateReceivedAtIndexes do
+  use Ecto.Migration
+
+  def up do
+    # Remove duplicate indexes on received_at column
+    # Keep only packets_received_at_idx which is the most general purpose
+    drop_if_exists index(:packets, [:received_at], name: :packets_received_at_asc_idx)
+    drop_if_exists index(:packets, [:received_at], name: :packets_recent_hour_idx)
+  end
+
+  def down do
+    # Recreate the removed indexes
+    create_if_not_exists index(:packets, [:received_at],
+                           name: :packets_received_at_asc_idx,
+                           comment: "Index for cleanup worker queries (ascending for old packets)"
+                         )
+
+    create_if_not_exists index(:packets, [:received_at],
+                           name: :packets_recent_hour_idx,
+                           comment: "Index for recent packet queries"
+                         )
+  end
+end