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/database/sql/sqlcgen/models.go

@@ -74,9 +74,27 @@ type CoverArt struct {
 	MimeType   string
 }
 
+type DownloadDownload struct {
+	ID               string
+	LibraryID        int64
+	Source           string
+	RequestID        sql.NullInt64
+	ReleaseMbid      sql.NullString
+	ReleaseGroupMbid sql.NullString
+	RecordingMbid    sql.NullString
+	Artist           string
+	Album            string
+	Query            string
+	Expected         string
+	State            string
+	Error            string
+	CreatedAt        time.Time
+	UpdatedAt        time.Time
+}
+
 type DownloadItem struct {
 	ID            string
-	RequestID     string
+	DownloadID    string
 	ProviderID    int64
 	TransportID   sql.NullInt64
 	ExternalID    string
@@ -102,24 +120,6 @@ type DownloadProvider struct {
 }
 
 type DownloadRequest struct {
-	ID               string
-	LibraryID        int64
-	Source           string
-	WantID           sql.NullInt64
-	ReleaseMbid      sql.NullString
-	ReleaseGroupMbid sql.NullString
-	RecordingMbid    sql.NullString
-	Artist           string
-	Album            string
-	Query            string
-	Expected         string
-	State            string
-	Error            string
-	CreatedAt        time.Time
-	UpdatedAt        time.Time
-}
-
-type DownloadWant struct {
 	ID          int64
 	Mbid        string
 	Entity      string