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/database.go

@@ -306,6 +306,14 @@ func applySchema(ctx context.Context, db *sql.DB) error {
 		return fmt.Errorf("could not apply migrations: %w", err)
 	}
 
+	// The download subsystem's Want/Request rename reuses table names
+	// (download_requests names a different table before and after), so
+	// it cannot be a plain sql/migrations file the way an ADD COLUMN
+	// migration can; see download_rename_migration.go for why.
+	if err := migrateDownloadRename(ctx, db); err != nil {
+		return fmt.Errorf("could not migrate download rename: %w", err)
+	}
+
 	return nil
 }