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/explore/artifactbuild.go

@@ -0,0 +1,87 @@
+package explore
+
+import (
+	"context"
+
+	"yellowjacket/backend/jobs"
+)
+
+// Fetching and merging the prebuilt catalog artifact.
+//
+// This is the whole catalog build on a user's machine.  The import that
+// derives the catalog from the MetaBrainz dumps lives behind the
+// `indexbuild` tag and runs only in CI (see dumpbuild_stub.go).
+
+// Artifact build stages, shown in the jobs panel.
+const (
+	artifactStageDownload = iota
+	artifactStageMerge
+)
+
+var artifactStageNames = [...]string{
+	"Downloading catalog",
+	"Merging catalog",
+}
+
+// tryCoreArtifact fetches and merges the prebuilt catalog.  Every
+// failure path is non-fatal by design: the caller falls back, and a
+// fresh install with no network still gets its own library in Explore.
+func (si *SearchIndex) tryCoreArtifact(ctx context.Context) error {
+	si.mu.Lock()
+	si.buildStatus = IndexStatus{
+		Building: true,
+		Tiers: []TierStatus{
+			{Name: artifactStageNames[artifactStageDownload], State: "pending"},
+			{Name: artifactStageNames[artifactStageMerge], State: "pending"},
+		},
+	}
+	si.mu.Unlock()
+	si.refreshStatusCounts()
+
+	fetcher, err := newArtifactFetcher(si)
+	if err != nil {
+		return err
+	}
+
+	si.setTierStatus(artifactStageNames[artifactStageDownload], "running", 0, 0)
+	si.logIndexJob(jobs.LevelInfo, "Fetching prebuilt catalog")
+
+	path, err := fetcher.fetch(ctx)
+	if err != nil {
+		si.setTierStatus(artifactStageNames[artifactStageDownload], "error", 0, 0)
+
+		return err
+	}
+
+	si.setTierStatus(artifactStageNames[artifactStageDownload], "complete", 0, 0)
+	si.setTierStatus(artifactStageNames[artifactStageMerge], "running", 0, 0)
+
+	if err := si.importCoreArtifact(ctx, path); err != nil {
+		si.setTierStatus(artifactStageNames[artifactStageMerge], "error", 0, 0)
+		si.removeArtifactFile(path)
+
+		return err
+	}
+
+	si.removeArtifactFile(path)
+	si.setTierStatus(artifactStageNames[artifactStageMerge], "complete", 0, 0)
+
+	si.mu.Lock()
+	si.buildStatus.Building = false
+	si.mu.Unlock()
+
+	// Fold the user's own library into the freshly-merged catalog:
+	// owned entities the artifact does not cover are inserted, and
+	// covered ones are flagged in_library.
+	si.PopulateLocalCrossReferences()
+	si.refreshStatusCounts()
+	si.scheduleChampionRebuild()
+
+	return nil
+}
+
+// artifactAlreadyMerged reports whether this index already carries a
+// merged artifact, so a restart does not re-download one.
+func (si *SearchIndex) artifactAlreadyMerged() bool {
+	return si.hasMeta(coreArtifactVersionKey)
+}