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/importer_test.go

@@ -117,9 +117,9 @@ func newImportFixture(t *testing.T, names ...string) importFixture {
 	}
 }
 
-func fourTrackRequest() Request {
-	return Request{
-		ID:          "req-1",
+func fourTrackDownload() Download {
+	return Download{
+		ID:          "dl-1",
 		LibraryID:   1,
 		ReleaseMBID: "mbid-1",
 		Artist:      "Radiohead",
@@ -145,7 +145,7 @@ func TestImportPlacesAndTagsFiles(t *testing.T) {
 
 	got, err := f.importer.Import(
 		context.Background(),
-		fourTrackRequest(),
+		fourTrackDownload(),
 		Result{Dir: f.dir, Files: f.files},
 		ImportOptions{LibraryRoot: f.root, WriteTags: true},
 	)
@@ -184,12 +184,12 @@ func TestImportTagsBeforeMoving(t *testing.T) {
 
 	f := newImportFixture(t, "01 - Airbag.flac")
 
-	req := fourTrackRequest()
-	req.Expected = req.Expected[:1]
+	dl := fourTrackDownload()
+	dl.Expected = dl.Expected[:1]
 
 	if _, err := f.importer.Import(
 		context.Background(),
-		req,
+		dl,
 		Result{Dir: f.dir, Files: f.files},
 		ImportOptions{LibraryRoot: f.root, WriteTags: true},
 	); err != nil {
@@ -227,7 +227,7 @@ func TestImportContinuesWhenTaggingFails(t *testing.T) {
 
 	got, err := f.importer.Import(
 		context.Background(),
-		fourTrackRequest(),
+		fourTrackDownload(),
 		Result{Dir: f.dir, Files: f.files},
 		ImportOptions{LibraryRoot: f.root, WriteTags: true},
 	)
@@ -251,7 +251,7 @@ func TestImportRejectsTooIncomplete(t *testing.T) {
 
 	_, err := f.importer.Import(
 		context.Background(),
-		fourTrackRequest(),
+		fourTrackDownload(),
 		Result{Dir: f.dir, Files: f.files},
 		ImportOptions{LibraryRoot: f.root, WriteTags: true},
 	)
@@ -273,7 +273,7 @@ func TestImportRejectsNoAudio(t *testing.T) {
 
 	_, err := f.importer.Import(
 		context.Background(),
-		fourTrackRequest(),
+		fourTrackDownload(),
 		Result{Dir: f.dir, Files: f.files},
 		ImportOptions{LibraryRoot: f.root, WriteTags: true},
 	)
@@ -297,7 +297,7 @@ func TestImportSkipsNonAudioFiles(t *testing.T) {
 
 	got, err := f.importer.Import(
 		context.Background(),
-		fourTrackRequest(),
+		fourTrackDownload(),
 		Result{Dir: f.dir, Files: f.files},
 		ImportOptions{LibraryRoot: f.root, WriteTags: true},
 	)
@@ -322,8 +322,8 @@ func TestImportNeverOverwrites(t *testing.T) {
 
 	f := newImportFixture(t, "01 - Airbag.flac")
 
-	req := fourTrackRequest()
-	req.Expected = req.Expected[:1]
+	dl := fourTrackDownload()
+	dl.Expected = dl.Expected[:1]
 
 	existing := filepath.Join(
 		f.root, "Radiohead", "OK Computer", "01 Airbag.flac",
@@ -339,7 +339,7 @@ func TestImportNeverOverwrites(t *testing.T) {
 
 	got, err := f.importer.Import(
 		context.Background(),
-		req,
+		dl,
 		Result{Dir: f.dir, Files: f.files},
 		ImportOptions{LibraryRoot: f.root, WriteTags: true},
 	)
@@ -366,12 +366,12 @@ func TestImportCustomPathTemplate(t *testing.T) {
 
 	f := newImportFixture(t, "01 - Airbag.flac")
 
-	req := fourTrackRequest()
-	req.Expected = req.Expected[:1]
+	dl := fourTrackDownload()
+	dl.Expected = dl.Expected[:1]
 
 	got, err := f.importer.Import(
 		context.Background(),
-		req,
+		dl,
 		Result{Dir: f.dir, Files: f.files},
 		ImportOptions{
 			LibraryRoot:  f.root,
@@ -423,12 +423,12 @@ func TestImportRequiresLibraryRoot(t *testing.T) {
 
 	f := newImportFixture(t, "01 - Airbag.flac")
 
-	req := fourTrackRequest()
-	req.Expected = req.Expected[:1]
+	dl := fourTrackDownload()
+	dl.Expected = dl.Expected[:1]
 
 	_, err := f.importer.Import(
 		context.Background(),
-		req,
+		dl,
 		Result{Dir: f.dir, Files: f.files},
 		ImportOptions{WriteTags: true},
 	)