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_test.go

@@ -327,7 +327,7 @@ func TestLidarrDelegateExistingAlbum(t *testing.T) {
 
 	l := newStubLidarr(t, stub)
 
-	externalID, err := l.Delegate(context.Background(), Request{
+	externalID, err := l.Delegate(context.Background(), Download{
 		ReleaseGroupMBID: "rg-mbid",
 		Artist:           "Radiohead",
 		Album:            "OK Computer",
@@ -373,7 +373,7 @@ func TestLidarrDelegateNewArtistMonitorsNothingByDefault(t *testing.T) {
 
 	l := newStubLidarr(t, stub)
 
-	if _, err := l.Delegate(context.Background(), Request{
+	if _, err := l.Delegate(context.Background(), Download{
 		ReleaseGroupMBID: "rg-mbid",
 		Artist:           "Radiohead",
 		Album:            "OK Computer",
@@ -414,7 +414,7 @@ func TestLidarrDelegateNoMatch(t *testing.T) {
 
 	l := newStubLidarr(t, stub)
 
-	_, err := l.Delegate(context.Background(), Request{
+	_, err := l.Delegate(context.Background(), Download{
 		Artist: "Nobody",
 		Album:  "Nothing",
 	})