HH-442: isolate runtime processes and harden shutdown (#90)

* HH-442: isolate runtime processes and harden shutdown

* HH-442: harden worker shutdown races

* HH-442: gate dependency shutdown on active handlers

---------

Co-authored-by: Rogee <rogee@ipao.vip>

backend/internal/lifecycle/handler_group.go

@@ -0,0 +1,63 @@
+package lifecycle
+
+import (
+	"context"
+	"sync"
+)
+
+// HandlerGroup stops new handlers, cancels their shared lifecycle context,
+// and waits for handlers already using application dependencies.
+type HandlerGroup struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+
+	mu      sync.Mutex
+	done    *sync.Cond
+	active  int
+	stopped bool
+}
+
+func NewHandlerGroup() *HandlerGroup {
+	ctx, cancel := context.WithCancel(context.Background())
+	g := &HandlerGroup{ctx: ctx, cancel: cancel}
+	g.done = sync.NewCond(&g.mu)
+	return g
+}
+
+func (g *HandlerGroup) Context() context.Context { return g.ctx }
+
+func (g *HandlerGroup) Begin() bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	if g.stopped {
+		return false
+	}
+	g.active++
+	return true
+}
+
+func (g *HandlerGroup) Done() {
+	g.mu.Lock()
+	g.active--
+	if g.active == 0 {
+		g.done.Broadcast()
+	}
+	g.mu.Unlock()
+}
+
+func (g *HandlerGroup) Stop() {
+	g.mu.Lock()
+	if !g.stopped {
+		g.stopped = true
+		g.cancel()
+	}
+	g.mu.Unlock()
+}
+
+func (g *HandlerGroup) Wait() {
+	g.mu.Lock()
+	for g.active != 0 {
+		g.done.Wait()
+	}
+	g.mu.Unlock()
+}