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

frontend/src/components/sidebar/app-sidebar.ts

@@ -5,7 +5,7 @@ import { designTokens } from '../../styles/tokens.css';
 
 import type { DragActiveDetail } from '@utils/drag-controller';
 
-type View = 'home' | 'playlists' | 'artists' | 'genres' | 'albums' | 'tracks' | 'explore' | 'wanted' | 'autotag' | 'jobs' | 'settings';
+type View = 'home' | 'playlists' | 'artists' | 'genres' | 'albums' | 'tracks' | 'explore' | 'downloads' | 'autotag' | 'jobs' | 'settings';
 
 interface NavItem {
     id: View;
@@ -149,7 +149,7 @@ export class AppSidebar extends LitElement {
         { id: 'albums', label: 'Albums', icon: 'compact-disc' },
         { id: 'tracks', label: 'Tracks', icon: 'music' },
         { id: 'explore', label: 'Explore', icon: 'globe' },
-        { id: 'wanted', label: 'Wanted', icon: 'bookmark' },
+        { id: 'downloads', label: 'Downloads', icon: 'bookmark' },
         { id: 'autotag', label: 'Autotag', icon: 'tag' },
         { id: 'jobs', label: 'Jobs', icon: 'list-check' },
         { id: 'settings', label: 'Settings', icon: 'gear' },