Surface a broken ju at app build time, not per request #8

Follow-up from the adversarial review of #6.

After #6, a broken ju install (e.g. a missing dependency of ju) raises instead of silently degrading. But FunctionSpecStore generates specs lazily, so mk_rjsf_app still builds successfully and every request (including / and /api/functions) then returns 500 with the import error, and routes.py converts it to JSON without the traceback.

Suggested: resolve _import_func_to_form_spec() once when the store or app is constructed, so a broken environment fails at startup with a full traceback.

Cosmetic, same area: JuNotInstalledWarning uses stacklevel=4, which assumes access through __getitem__. Through Mapping.get / .items() (used by routes.py) it is attributed to <frozen _collections_abc>. Resolving at construction time fixes that too.