fix: correct multiple server-side bugs in handlers and model client

[wishhyt]

Summary

Fix 4 server-side bugs:

• InviteUserHandler (invites.go:81-86): Two bugs in domain check: (1) compared
  pointer addresses instead of string values (org.Domain == domain where both are
  *string), making the check always fail; (2) missing return after http.Error(),
  allowing execution to continue and potentially create duplicate invites.
• CreateChatCompletionWithInternalStream (client_stream.go:43-44): Unconditionally
  appended :nitro to OpenRouter model names without checking for existing suffixes.
  The non-streaming CreateChatCompletionStream in client.go correctly checks for
  :nitro, :free, and :floor suffixes. Added the same check to prevent invalid
  model names like model:free:nitro.
• AddPlanConvoMessage (plan_helpers.go:178-182): Missing return after sending
  error to errCh. When Exec failed, both an error and a nil were sent (2 writes),
  but combined with the first goroutine's write (1 write), that's 3 writes to a channel
  read only 2 times — the third write blocks forever, leaking the goroutine.
• ListPlansRunningHandler (plans_crud.go:544-549): Sort function used composite
  key PlanId + "|" + Name to look up StreamFinishedAtByBranchId map, but the map
  was keyed by branch.Id. Lookups always missed, making the sort ineffective.

Test plan

• Verify invite domain check correctly prevents duplicate invites
• Verify OpenRouter models with :free or :floor suffix work in streaming mode
• Verify no goroutine leaks when plan reply count update fails
• Verify running plans list is correctly sorted by stream status