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/config.go

@@ -33,6 +33,34 @@ type UserConfig struct {
 	// for.  A large list should be worked through steadily rather than
 	// in one burst that every provider sees as a flood.
 	WantedBatch int `toml:"WantedBatch"`
+
+	// MinFileSizeMB, MaxFileSizeMB and PreferredFileSizeMB bound and
+	// nudge what auto-pick (interactive or via the request list) may
+	// grab without asking.  Zero on any of them is permissive: see
+	// AutoDownloadPrefs.
+	MinFileSizeMB       int `toml:"MinFileSizeMB"`
+	MaxFileSizeMB       int `toml:"MaxFileSizeMB"`
+	PreferredFileSizeMB int `toml:"PreferredFileSizeMB"`
+
+	// AllowedFormats restricts auto-pick to these formats.  Empty means
+	// no restriction.  Values are Format strings ("flac", "mp3", ...).
+	AllowedFormats []string `toml:"AllowedFormats"`
+}
+
+// AutoDownloadPrefs converts the persisted guardrail fields to the
+// runtime type Manager and the ranker consume.
+func (c *UserConfig) AutoDownloadPrefs() AutoDownloadPrefs {
+	formats := make([]Format, 0, len(c.AllowedFormats))
+	for _, f := range c.AllowedFormats {
+		formats = append(formats, Format(f))
+	}
+
+	return AutoDownloadPrefs{
+		MinSizeMB:       c.MinFileSizeMB,
+		MaxSizeMB:       c.MaxFileSizeMB,
+		PreferredSizeMB: c.PreferredFileSizeMB,
+		AllowedFormats:  formats,
+	}
 }
 
 // ApplyDefaults fills unset fields.