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/concurrency_test.go

@@ -84,17 +84,17 @@ func TestPerProviderCapSerializesTransfers(t *testing.T) {
 
 	// Three requests against the same one-at-a-time provider.
 	for i := range 3 {
-		req := fourTrackRequest()
-		req.ID = "req-" + string(rune('a'+i))
+		dl := fourTrackDownload()
+		dl.ID = "dl-" + string(rune('a'+i))
 
-		if err := f.store.CreateRequest(ctx, req); err != nil {
-			t.Fatalf("CreateRequest: %v", err)
+		if err := f.store.CreateDownload(ctx, dl); err != nil {
+			t.Fatalf("CreateDownload: %v", err)
 		}
 
 		candidate := slow.Candidates[0]
 		candidate.ProviderID = 1
 
-		go f.manager.grab(ctx, req, candidate, nil)
+		go f.manager.grab(ctx, dl, candidate, nil)
 	}
 
 	// Give all three a chance to reach the transport, then check how
@@ -139,17 +139,17 @@ func TestPerProviderCapAllowsParallelWhereSafe(t *testing.T) {
 	ctx := context.Background()
 
 	for i := range 3 {
-		req := fourTrackRequest()
-		req.ID = "req-" + string(rune('a'+i))
+		dl := fourTrackDownload()
+		dl.ID = "dl-" + string(rune('a'+i))
 
-		if err := f.store.CreateRequest(ctx, req); err != nil {
-			t.Fatalf("CreateRequest: %v", err)
+		if err := f.store.CreateDownload(ctx, dl); err != nil {
+			t.Fatalf("CreateDownload: %v", err)
 		}
 
 		candidate := fast.Candidates[0]
 		candidate.ProviderID = 1
 
-		go f.manager.grab(ctx, req, candidate, nil)
+		go f.manager.grab(ctx, dl, candidate, nil)
 	}
 
 	waitFor(