Add covering index for encoding queue filtering

Create partial index on videos(state, priority DESC, updated_at DESC)
for state='crf_searched' to help with filtering and sorting before
joining to vmafs. Reduces unnecessary temp sorts on the videos side.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: mjc

priv/repo/migrations/20260319175802_add_encoding_queue_index.exs

@@ -0,0 +1,17 @@
+defmodule Reencodarr.Repo.Migrations.AddEncodingQueueIndex do
+  use Ecto.Migration
+
+  def up do
+    # Covering index for encoding queue (videos ready for encoding with state filter)
+    # Helps with filtering and ordering by priority/updated_at before joining to vmafs
+    execute("""
+    CREATE INDEX videos_encoding_queue_index
+    ON videos (state, priority DESC, updated_at DESC)
+    WHERE state = 'crf_searched'
+    """)
+  end
+
+  def down do
+    execute("DROP INDEX IF EXISTS videos_encoding_queue_index")
+  end
+end