Replace synchronous RPC-based CP-host communication for sandbox
lifecycle operations (Create, Pause, Resume, Destroy) with an async
pattern. CP handlers now return 202 Accepted immediately, fire agent
RPCs in background goroutines, and publish state events to a Redis
Stream. A background consumer processes events as a fallback writer.
Agent-side auto-pause events are pushed to the CP via HTTP callback
(POST /v1/hosts/sandbox-events), keeping Redis internal to the CP.
All DB status transitions use conditional updates
(UpdateSandboxStatusIf, UpdateSandboxRunningIf) to prevent race
conditions between concurrent operations and background goroutines.
The HostMonitor reconciler is kept at 60s as a safety net, extended
to handle transient statuses (starting, pausing, resuming, stopping).
Frontend updated to handle 202 responses with empty bodies and render
transient statuses with blue indicators.
Moves 12 packages from internal/ to pkg/ (config, id, validate, events, db,
auth, lifecycle, scheduler, channels, audit, service) so they can be imported
by the enterprise repo as a Go module dependency.
Introduces pkg/cpextension (shared Extension interface + ServerContext) and
pkg/cpserver (Run() entrypoint with functional options) so the enterprise
main.go can call cpserver.Run(cpserver.WithExtensions(...)) without duplicating
the 20-step server bootstrap. Adds db/migrations/embed.go for go:embed access
to OSS SQL migrations from the enterprise module.
cmd/control-plane/main.go is reduced to a 10-line wrapper around cpserver.Run.
