feat(jobs): surface background jobs with progress, logs and controls

Add a central job registry that library scans and search index builds
report into, so background work is visible instead of buried in the
settings page.

- backend/jobs: registry with per-job ring-buffer logs, capability-driven
  controls, and one coalesced JobsChanged snapshot at 4Hz
- pause survives restart via a job_state table; a paused scan is adopted
  back on launch and skipped by the soft scan
- top-bar indicator, popover, details drawer and a Jobs page replacing
  the config page's scan UI; per-library start/stop retained
- scan timing breakdown moves into the job log, Full rescan to the Jobs
  page; delete the orphaned library-manager component

Also add cmd/indexbuild and cmd/indexexport so the explore index can be
built once centrally rather than by every install, which today streams
~205GB from the ListenBrainz spark dump on first run. indexbuild picks
build/refresh/rebuild from index state; the Gitea workflow runs it on
push, weekly, or manually and publishes only when content changed.

fresh-install no longer defaults YJ_HOME under /tmp: it is tmpfs on most
distros, and the import needs ~6GB of real disk.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/system/userdata_test.go

@@ -0,0 +1,52 @@
+package system
+
+import (
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestResolveUserDirPath_HomeOverride(t *testing.T) {
+	home := t.TempDir()
+	t.Setenv(envHomeOverride, home)
+
+	tests := []struct {
+		name string
+		dt   dirType
+		want string
+	}{
+		{name: "config", dt: dirTypeConfig, want: filepath.Join(home, "config")},
+		{name: "data", dt: dirTypeData, want: filepath.Join(home, "data")},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := resolveUserDirPath(tt.dt)
+			if err != nil {
+				t.Fatalf("resolveUserDirPath(%q) returned error: %v", tt.dt, err)
+			}
+
+			if got != tt.want {
+				t.Errorf("resolveUserDirPath(%q) = %q, want %q", tt.dt, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestResolveUserDirPath_NoOverrideUsesOSPath(t *testing.T) {
+	t.Setenv(envHomeOverride, "")
+
+	got, err := resolveUserDirPath(dirTypeConfig)
+	if err != nil {
+		t.Fatalf("resolveUserDirPath returned error: %v", err)
+	}
+
+	// Without the override the path must fall back to the OS-specific
+	// yellowjacket location, not a bare "<home>/config" base dir.
+	if !filepath.IsAbs(got) || !strings.HasSuffix(got, "yellowjacket") {
+		t.Errorf(
+			"resolveUserDirPath fallback = %q, want absolute path ending in %q",
+			got, "yellowjacket",
+		)
+	}
+}