Add auto-pause TTL and ping endpoint for sandbox inactivity management

Replace the existing auto-destroy TTL behavior with auto-pause: when a
sandbox exceeds its timeout_sec of inactivity, the TTL reaper now pauses
it (snapshot + teardown) instead of destroying it, preserving the ability
to resume later.

Key changes:
- TTL reaper calls Pause instead of Destroy, with fallback to Destroy if
  pause fails (e.g. Firecracker process already gone)
- New PingSandbox RPC resets the in-memory LastActiveAt timer
- New POST /v1/sandboxes/{id}/ping REST endpoint resets both agent memory
  and DB last_active_at
- ListSandboxes RPC now includes auto_paused_sandbox_ids so the reconciler
  can distinguish auto-paused sandboxes from crashed ones in a single call
- Reconciler polls every 5s (was 30s) and marks auto-paused as "paused"
  vs orphaned as "stopped"
- Resume RPC accepts timeout_sec from DB so TTL survives pause/resume cycles
- Reaper checks every 2s (was 10s) and uses a detached context to avoid
  incomplete pauses on app shutdown
- Default timeout_sec changed from 300 to 0 (no auto-pause unless requested)

internal/api/server.go

@@ -64,6 +64,7 @@ func New(queries *db.Queries, agent hostagentv1connect.HostAgentServiceClient, p
 			r.Delete("/", sandbox.Destroy)
 			r.Post("/exec", exec.Exec)
 			r.Get("/exec/stream", execStream.ExecStream)
+			r.Post("/ping", sandbox.Ping)
 			r.Post("/pause", sandbox.Pause)
 			r.Post("/resume", sandbox.Resume)
 			r.Post("/files/write", files.Upload)