api: add pagination (limit/offset) and downloaded filter to media endpoint

The /api/v1/media endpoint was loading ALL media items from the database
with no pagination, causing timeouts with large libraries (1800+ items
per source). Now supports:
- ?limit=N (default 50)
- ?offset=N (default 0)
- ?downloaded=true (filter to only downloaded media)
- Ordered by uploaded_at desc

lib/pinchflat_web/controllers/api/v1/api_media_item_controller.ex

@@ -12,13 +12,17 @@ defmodule PinchflatWeb.Api.V1.ApiMediaItemController do
   alias Pinchflat.Downloading.MediaDownloadWorker
 
   def index(conn, params) do
+    limit = parse_param(params, "limit", 50)
+    offset = parse_param(params, "offset", 0)
+    downloaded_only = params["downloaded"] == "true"
+
     media_items =
       case params do
         %{"source_id" => source_id} ->
-          Media.list_media_items_for_source(source_id)
+          Media.list_media_items_for_source(source_id, limit: limit, offset: offset, downloaded_only: downloaded_only)
 
         _ ->
-          Media.list_media_items()
+          Media.list_media_items(limit: limit, offset: offset, downloaded_only: downloaded_only)
       end
 
     media_items = Repo.preload(media_items, :source)
@@ -93,4 +97,15 @@ defmodule PinchflatWeb.Api.V1.ApiMediaItemController do
       end)
     end)
   end
+
+  defp parse_param(params, key, default) do
+    case Map.get(params, key) do
+      nil -> default
+      value ->
+        case Integer.parse(value) do
+          {n, _} -> n
+          :error -> default
+        end
+    end
+  end
 end