refactor: make Filter service stateless

[Copilot]

Filter held a private array $forms keyed by $name . $sessionId, accumulating form instances indefinitely on the service object. In long-running processes (FPM workers, Messenger consumers) this leaks memory and can bleed state across requests.

Changes

• Removed $forms instance variable — service now has zero mutable state
• Session-backed metadata — saveFilter() writes filter_meta.{name} to the session containing the form class and options; forms are recreated on demand instead of cached
• filter() — replaces $form->isSubmitted() with $request->query->has('submit-filter') (logically equivalent: the old form was only submitted when saveFilter() called handleRequest(), which itself gated on that param)
• getForm() — reads filter_meta.{name} from session, recreates the form with the stored type/options, calls handleRequest() only when submit-filter is present so getFormView() still surfaces validation errors correctly; guards against missing metadata with an early return

Session keys now written

Key	Content
filter.{name}	filter values (unchanged)
filter_sort.{name}	sort info (unchanged)
filter_meta.{name}	{type, options} — new