feat(cli): upgrade command, rich status, and version cache

- Replace `upgrade` stub with real command (alias for `self-update`)
- Also register `update` as alias: `unarr update` works too
- Rewrite `status` to show full config, disk usage, daemon state, and
  update availability with colored sections
- Add version check cache (1h TTL) so `status` is instant on repeat runs
- Guard against division by zero on empty filesystems
- Guard against negative durations from clock skew
- Guard against stale PID via heartbeat recency check (2 min)
- Add comprehensive test coverage across agent, engine, upgrade, usenet,
  arr, library, mediaserver, and UI packages
- Improve Makefile coverage target to exclude cmd/ glue code
- Fix stream handler resource cleanup and ffprobe error handling

Makefile

@@ -19,10 +19,13 @@ test:
 lint:
 	golangci-lint run ./...
 
-## Run tests with coverage report
+## Run tests with coverage report (excludes CLI layer — cmd/ is glue code)
+COVER_PKGS = $(shell go list ./... | grep -v '/cmd')
 coverage:
-	go test -race -coverprofile=coverage.out -covermode=atomic ./...
-	go tool cover -func=coverage.out
+	go test -race -coverprofile=coverage.out -covermode=atomic $(COVER_PKGS)
+	@echo "──────────────────────────────────────"
+	@go tool cover -func=coverage.out | tail -1
+	@echo "──────────────────────────────────────"
 	go tool cover -html=coverage.out -o coverage.html
 
 ## Format code