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_prowlarr_test.go

@@ -169,7 +169,7 @@ func TestProwlarrSearchMarksProtocols(t *testing.T) {
 
 	p := newStubProwlarr(t, stub, nil)
 
-	got, err := p.Search(context.Background(), Request{
+	got, err := p.Search(context.Background(), Download{
 		Artist: "Radiohead",
 		Album:  "OK Computer",
 	})
@@ -230,7 +230,7 @@ func TestProwlarrFiltersDeadTorrents(t *testing.T) {
 
 	p := newStubProwlarr(t, stub, map[string]string{"minSeeders": "1"})
 
-	got, err := p.Search(context.Background(), Request{Query: "x"})
+	got, err := p.Search(context.Background(), Download{Query: "x"})
 	if err != nil {
 		t.Fatalf("Search: %v", err)
 	}
@@ -253,7 +253,7 @@ func TestProwlarrSearchesMusicCategory(t *testing.T) {
 	p := newStubProwlarr(t, stub, nil)
 
 	if _, err := p.Search(
-		context.Background(), Request{Query: "radiohead"},
+		context.Background(), Download{Query: "radiohead"},
 	); err != nil {
 		t.Fatalf("Search: %v", err)
 	}