fix(api): track background goroutines so shutdown drains in-flight events

[AdaAibaby]

Summary

Fixes #3357.

The API shutdown path waited for HTTP/gRPC handlers to return but silently abandoned background goroutines spawned during request handling (analytics events, volume cleanup). This PR introduces a sync.WaitGroup on Orchestrator to track those goroutines and waits for them with a 30-second budget after the HTTP drain completes.

What is tracked

Goroutine	Site	Why it matters
handleNewlyCreatedSandbox	sandbox/store.go	analytics insert + posthog event per sandbox created
analyticsRemove (x2)	orchestrator/delete_instance.go	billing-critical analytics event per sandbox deleted
removeSandboxFromNode cleanup	orchestrator/create_instance.go	kills orphaned sandboxes on store-add failure
deleteVolume	handlers/volume_delete.go, volume_create.go	removes volume data from orchestrator node

What is intentionally NOT tracked

• BuildStatusSync in template-manager/create_template.go — can run up to 1 hour; cancellation via context is the correct shutdown signal for long-running polling loops.
• Lifecycle goroutines (keepInSync, startStatusLogging, etc.) — managed by the root context and terminate when it is cancelled.

New API surface

// Orchestrator
func (o *Orchestrator) GoBackground(fn func())              // replaces bare "go"
func (o *Orchestrator) WaitBackground(ctx context.Context) bool

// APIStore (delegates to orchestrator)
func (a *APIStore) GoBackground(fn func())
func (a *APIStore) WaitBackground(ctx context.Context) bool

Shutdown sequence (after)

signal -> 15s health drain -> HTTP+gRPC parallel drain (up to 75s)
       -> background goroutine drain (up to 30s, new)
       -> cleanup()

Test plan

• 4 new unit tests in orchestrator/background_wg_test.go:
  • TestGoBackgroundTracksGoroutine — goroutine is awaited
  • TestWaitBackgroundTimesOutOnSlow — ctx expiry returns false
  • TestGoBackgroundMultipleGoroutines — N concurrent goroutines all drained
  • TestWaitBackgroundNoGoroutinesReturnsImmediately — zero-goroutine fast path
• go build ./packages/api/... clean
• All existing ./packages/api/... tests pass

/cc @jakubno @dobrac @ValentaTomas @arkamar @tvi Looking forward to your code review.