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/testfixtures_test.go

@@ -0,0 +1,108 @@
+package explore
+
+import (
+	"archive/tar"
+	"bytes"
+	"log/slog"
+	"testing"
+
+	"github.com/parquet-go/parquet-go"
+)
+
+// Fixtures shared by the client tests and the tagged index-builder
+// tests.  They live in an untagged file so both builds compile.
+
+// Fixed MBIDs for fixtures.
+const (
+	recA = "11111111-1111-1111-1111-111111111111"
+	recB = "22222222-2222-2222-2222-222222222222"
+	recC = "33333333-3333-3333-3333-333333333333"
+	relA = "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
+	relB = "bbbbbbbb-bbbb-bbbb-bbbb-bbbbbbbbbbbb"
+	rgA  = "cccccccc-cccc-cccc-cccc-cccccccccccc"
+	rgB  = "dddddddd-dddd-dddd-dddd-dddddddddddd"
+	artA = "eeeeeeee-eeee-eeee-eeee-eeeeeeeeeeee"
+	artB = "ffffffff-ffff-ffff-ffff-ffffffffffff"
+)
+
+func testLogger() *slog.Logger {
+	return slog.New(slog.DiscardHandler)
+}
+
+func makeTar(t *testing.T, members map[string][]byte, order []string) []byte {
+	t.Helper()
+
+	var buf bytes.Buffer
+
+	tw := tar.NewWriter(&buf)
+
+	for _, name := range order {
+		data := members[name]
+		hdr := &tar.Header{
+			Name:     name,
+			Mode:     0o644,
+			Size:     int64(len(data)),
+			Typeflag: tar.TypeReg,
+		}
+
+		if err := tw.WriteHeader(hdr); err != nil {
+			t.Fatalf("tar header: %v", err)
+		}
+
+		if _, err := tw.Write(data); err != nil {
+			t.Fatalf("tar write: %v", err)
+		}
+	}
+
+	if err := tw.Close(); err != nil {
+		t.Fatalf("tar close: %v", err)
+	}
+
+	return buf.Bytes()
+}
+
+func makeParquet(t *testing.T, rows []sparkFixtureRow) []byte {
+	t.Helper()
+
+	var buf bytes.Buffer
+
+	w := parquet.NewGenericWriter[sparkFixtureRow](&buf)
+
+	if _, err := w.Write(rows); err != nil {
+		t.Fatalf("parquet write: %v", err)
+	}
+
+	if err := w.Close(); err != nil {
+		t.Fatalf("parquet close: %v", err)
+	}
+
+	return buf.Bytes()
+}
+
+// sparkFixtureRow mimics the real spark listens schema: the aggregator
+// must project just recording/release/artist MBIDs out of it.
+type sparkFixtureRow struct {
+	ListenedAt    int64    `parquet:"listened_at"`
+	UserID        int64    `parquet:"user_id"`
+	ArtistName    string   `parquet:"artist_name,optional"`
+	RecordingMBID string   `parquet:"recording_mbid,optional"`
+	ReleaseMBID   string   `parquet:"release_mbid,optional"`
+	ArtistMBIDs   []string `parquet:"artist_credit_mbids,optional,list"`
+}
+
+// listensOf builds n identical listen rows for a recording.
+func listensOf(n int, recording, release string, artists []string) []sparkFixtureRow {
+	rows := make([]sparkFixtureRow, n)
+	for i := range rows {
+		rows[i] = sparkFixtureRow{
+			ListenedAt:    1700000000 + int64(i),
+			UserID:        int64(i),
+			ArtistName:    "Fixture Artist",
+			RecordingMBID: recording,
+			ReleaseMBID:   release,
+			ArtistMBIDs:   artists,
+		}
+	}
+
+	return rows
+}