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>

Makefile

@@ -2,11 +2,39 @@ VERSION ?= dev
 COMMIT  ?= $(shell git rev-parse --short HEAD 2>/dev/null || echo "unknown")
 LDFLAGS := -X 'main.version=$(VERSION)' -X 'main.commit=$(COMMIT)'
 
+# YJ_HOME isolates the dev build's config + database from a packaged
+# install. Defaults to a sandbox under XDG data; override in .env to
+# point elsewhere (or unset it there to share the real user dirs).
+DEV_YJ_HOME ?= $(HOME)/.local/share/yellowjacket-dev
+
 dev: setup generate clean
-	if [ -f .env ]; then set -a; . ./.env; set +a; fi; go tool wails dev -tags webkit2_41 -loglevel Debug -v 2
+	if [ -f .env ]; then set -a; . ./.env; set +a; fi; : "$${YJ_HOME:=$(DEV_YJ_HOME)}"; export YJ_HOME; go tool wails dev -tags webkit2_41 -loglevel Debug -v 2
 
 dev-debug: setup generate clean
-	if [ -f .env ]; then set -a; . ./.env; set +a; fi; YJ_LOG_LEVEL=debug go tool wails dev -tags webkit2_41 -loglevel Debug -v 2
+	if [ -f .env ]; then set -a; . ./.env; set +a; fi; : "$${YJ_HOME:=$(DEV_YJ_HOME)}"; export YJ_HOME; YJ_LOG_LEVEL=debug go tool wails dev -tags webkit2_41 -loglevel Debug -v 2
+
+# Base directory for fresh-install sandboxes. Deliberately NOT $TMPDIR:
+# on most Linux distros /tmp is tmpfs (RAM-backed) and only a few GB, so
+# the search index dump import — which wants 6GB free before it will even
+# start, then streams multi-GB dumps through explore-staging/ — either
+# fails its precheck or eats that much RAM. XDG cache is disk-backed
+# everywhere and still throwaway.
+FRESH_HOME_BASE ?= $(if $(XDG_CACHE_HOME),$(XDG_CACHE_HOME),$(HOME)/.cache)
+
+# fresh-install runs dev against a brand-new YJ_HOME so every launch
+# starts from a clean first-run state (no config.toml, no yj.db). The dir
+# is not cleaned up automatically, so you can inspect it afterward; the
+# printed path tells you where it is. Override the location with
+# FRESH_HOME_BASE=/some/disk make fresh-install.
+fresh-install: setup generate clean
+	if [ -f .env ]; then set -a; . ./.env; set +a; fi; \
+	mkdir -p "$(FRESH_HOME_BASE)"; \
+	export YJ_HOME="$$(mktemp -d "$(FRESH_HOME_BASE)/yellowjacket-fresh.XXXXXX")"; \
+	echo "==> fresh YJ_HOME=$$YJ_HOME"; \
+	case "$$(findmnt -no FSTYPE -T "$$YJ_HOME" 2>/dev/null)" in \
+	  tmpfs|ramfs) echo "==> WARNING: $$YJ_HOME is RAM-backed; the search index import needs ~6GB of real disk. Set FRESH_HOME_BASE to a disk-backed path." ;; \
+	esac; \
+	go tool wails dev -tags webkit2_41 -loglevel Debug -v 2
 
 build-dev: generate
 	go tool wails build -tags webkit2_41 -debug -clean -ldflags "$(LDFLAGS)"