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/index.ts

@@ -23,7 +23,7 @@ import '@components/autotag-view/autotag-view.ts';
 import '@components/first-run-wizard/first-run-wizard.ts';
 import '@components/jobs/job-indicator.ts';
 import '@components/jobs/jobs-view.ts';
-import '@components/wanted-view/wanted-view.ts';
+import '@components/downloads-view/downloads-view.ts';
 import '@awesome.me/webawesome/dist/styles/themes/default.css';
 import '@awesome.me/webawesome/dist/components/icon/icon.js';
 import { setBasePath } from '@awesome.me/webawesome/dist/webawesome.js';
@@ -64,7 +64,7 @@ const VIEW_TAGS: Record<string, string> = {
     playlists: 'playlist-view',
     explore: 'explore-view',
     autotag: 'autotag-view',
-    wanted: 'wanted-view',
+    downloads: 'downloads-view',
     jobs: 'jobs-view',
     settings: 'config-page',
 };