feat: data lifecycle rewrite, download clients, wanted list, and central catalog index

Ships the fresh-start schema cleanup: rebuilt explore catalog index
pipeline (dump import, artifact fetch/build, incremental listen-count
refresh), a new download subsystem (Lidarr/Prowlarr/qBittorrent/SABnzbd/
slskd/yt-dlp providers, staging, reconciliation, wanted list), and the
supporting schema/query/store changes across backend and frontend.

Also includes two smaller follow-ups: bump the central index's
rebuild-after cadence from 90 to 180 days, and remove the Explore
"library only" online/offline toggle entirely (frontend-only, no
backend counterpart) rather than carry unused UI/state.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Agd9af5hE7qzti2ackiS

backend/download/config.go

@@ -0,0 +1,60 @@
+package download
+
+import "time"
+
+// UserConfig is the download subsystem's slice of the TOML config file.
+// Provider connections are not here — they live in the database, keyed
+// by row, because there can be many of them and they change through the
+// settings UI rather than by hand-editing.
+type UserConfig struct {
+	// PathTemplate lays out imported files under the library root.
+	// Tokens: {albumartist} {artist} {album} {year} {track} {disc}
+	// {title}.  Empty falls back to DefaultPathTemplate.
+	PathTemplate string `toml:"PathTemplate"`
+
+	// AutoPick lets a single high-confidence, high-quality candidate
+	// download without asking.  Off by default: an unattended download
+	// that picks wrong puts the wrong files in the library, and the
+	// ranking has to earn that trust on a given user's sources first.
+	AutoPick bool `toml:"AutoPick"`
+
+	// MaxConcurrent bounds simultaneous transfers across all providers.
+	// Per-provider limits sit underneath it and are set on the provider
+	// itself, since the right number depends on what is on the other
+	// end: one Soulseek peer, or a usenet server built for parallelism.
+	MaxConcurrent int `toml:"MaxConcurrent"`
+
+	// WantedIntervalMinutes is how often the wanted list is reconciled:
+	// artist subscriptions expanded, owned items retired, due wants
+	// searched for.  Zero uses the default.
+	WantedIntervalMinutes int `toml:"WantedIntervalMinutes"`
+
+	// WantedBatch bounds how many wants one reconcile pass searches
+	// 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"`
+}
+
+// ApplyDefaults fills unset fields.
+func (c *UserConfig) ApplyDefaults() {
+	if c.PathTemplate == "" {
+		c.PathTemplate = DefaultPathTemplate
+	}
+
+	if c.MaxConcurrent <= 0 {
+		c.MaxConcurrent = defaultConcurrency
+	}
+
+	if c.WantedIntervalMinutes <= 0 {
+		c.WantedIntervalMinutes = int(defaultReconcileInterval / time.Minute)
+	}
+
+	if c.WantedBatch <= 0 {
+		c.WantedBatch = defaultDueBatch
+	}
+}
+
+// WantedInterval is the reconcile interval as a duration.
+func (c *UserConfig) WantedInterval() time.Duration {
+	return time.Duration(c.WantedIntervalMinutes) * time.Minute
+}