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/download/provider_lidarr.go

@@ -237,8 +237,8 @@ type lidarrTrackFile struct {
 // Delegate adds the album to Lidarr, monitors it and triggers a search.
 // The external ID returned is Lidarr's album ID, which is what Poll
 // needs and what survives a restart.
-func (l *lidarr) Delegate(ctx context.Context, req Request) (string, error) {
-	album, err := l.findAlbum(ctx, req)
+func (l *lidarr) Delegate(ctx context.Context, dl Download) (string, error) {
+	album, err := l.findAlbum(ctx, dl)
 	if err != nil {
 		return "", err
 	}
@@ -276,12 +276,12 @@ func (l *lidarr) Delegate(ctx context.Context, req Request) (string, error) {
 // not.
 func (l *lidarr) findAlbum(
 	ctx context.Context,
-	req Request,
+	dl Download,
 ) (lidarrAlbum, error) {
-	term := req.SearchText()
+	term := dl.SearchText()
 
-	if req.ReleaseGroupMBID != "" {
-		term = "lidarr:" + req.ReleaseGroupMBID
+	if dl.ReleaseGroupMBID != "" {
+		term = "lidarr:" + dl.ReleaseGroupMBID
 	}
 
 	var results []struct {
@@ -300,7 +300,7 @@ func (l *lidarr) findAlbum(
 		}
 	}
 
-	return lidarrAlbum{}, fmt.Errorf("%w: %s", ErrLidarrNoMatch, req.SearchText())
+	return lidarrAlbum{}, fmt.Errorf("%w: %s", ErrLidarrNoMatch, dl.SearchText())
 }
 
 // addArtistForAlbum adds the album's artist so the album becomes a real