refactor(download): rename Want/Request to Request/Download, unify downloads flow, add auto-download guardrails

The durable "I asked for this" record was called Want, and the one-shot
search-and-grab attempt was called Request — names that didn't match
what either actually did. Want is now Request, and the old Request/Item
is now Download/DownloadItem, with a table-rename migration
(download_wants -> download_requests, old download_requests ->
download_downloads) safe against both fresh installs and existing data.

Every anchored manual download now upserts/reuses a durable Request
before running, so a "download now" that finds nothing is picked up by
the background reconciler automatically instead of just failing with
no trace — the gap that caused this session's repeated "no candidates
found" failures on the same album.

Also adds auto-download guardrails (file-size min/max with a preferred
target, allowed file types) that gate what the pipeline may grab
unattended, live-editable from a new settings section. The frontend's
wanted-view becomes downloads-view, with a new Downloads tab showing
attempt/transfer history that previously had no UI at all.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Agd9af5hE7qzti2ackiS

backend/app.go

@@ -273,6 +273,7 @@ func (yj *YellowJacketApp) initDownloadRuntime(ctx context.Context) {
 		PathTemplate: cfg.PathTemplate,
 	})
 	yj.downloads.SetMaxConcurrent(cfg.MaxConcurrent)
+	yj.downloads.SetPreferences(cfg.AutoDownloadPrefs())
 
 	go func() {
 		if err := yj.downloads.Reload(ctx); err != nil {
@@ -289,7 +290,7 @@ func (yj *YellowJacketApp) initDownloadRuntime(ctx context.Context) {
 	yj.wanted.SetInterval(cfg.WantedInterval())
 	yj.wanted.SetBatch(cfg.WantedBatch)
 	yj.wanted.SetOnChange(func() {
-		wailsruntime.EventsEmit(ctx, events.WantedListChanged)
+		wailsruntime.EventsEmit(ctx, events.RequestsChanged)
 	})
 	yj.wanted.Start(ctx)
 }