Remove session-based status tracking that was storing crypto_payment_status_{payment_id}
for every payment, causing cookie size to exceed 4KB limit with 35+ payments.
- Remove: request.session[session_key] = current_status
- Remove: previous_status tracking via session
- Keep: Flash messages for status notifications (they don't bloat cookies)
Flash messages now trigger on every poll instead of just transitions, but this
prevents the cookie bloat issue. Client-side can handle transition detection if needed.