Fix Catalogue tab 500ing on first load, and declare rustler explicitly for mdex_native

[timujinne]

Two independent fixes on top of upstream/main.

Fix Catalogue tab 500ing on first load for every company

column_picker_available? and catalogue_column_catalog() were only computed in put_catalogue_columns/2, reached from the show/hide/update column-picker event handlers — never from handle_params. The template references @column_picker_available and @catalogue_column_catalog unconditionally whenever tab == "catalogue", so the very first render (the only way to reach those event handlers in the first place) raised KeyError before a user could ever trigger them.

Confirmed live: GET /admin/crm/companies/:uuid?tab=catalogue 500s on every company. Not something PR #75's soft-dependency fix touches; predates it (last touched in 97a086e). Covered by a point test, 5/5 passing.

Declare rustler explicitly so mdex_native force-builds

MDEX_NATIVE_BUILD=1 forces mdex_native to build its NIF from source when no precompiled build exists for the running OTP (28 in our case), which requires rustler itself, not just rustler_precompiled. mdex arrives here transitively through phoenix_kit; a clean checkout fails to compile without this. Same declaration phoenix_kit's own mix.exs already carries.

---

One fix, six repositories. Every module that pulls mdex transitively through phoenix_kit needs the same one-line declaration, and each repository needs its own PR. Already merged: BeamLabEU/phoenix_kit_catalogue#77, BeamLabEU/phoenix_kit_billing#27. Still open: BeamLabEU/phoenix_kit_warehouse#20, BeamLabEU/phoenix_kit_entities#35, BeamLabEU/phoenix_kit_document_creator#42.

Scope. This makes this repository build from a clean checkout under MDEX_NATIVE_BUILD=1. It does not remove the need for a host application to declare rustler itself: optional dependencies of dependencies are never resolved, so whichever project is the root has to declare it.

[timujinne]

Ran the full suite on this branch tip: mix deps.get, mix compile under MIX_ENV=test, then mix test with the :integration half enabled against a real Postgres instance, with MDEX_NATIVE_BUILD=1 set under OTP 28 — the exact scenario the rustler declaration in this PR targets. The declaration compiles and builds the native NIF cleanly in that environment.

618 tests, 0 failures (9 individually skipped, unrelated to this PR).

The Catalogue-tab fix is additionally covered by its own point test mentioned in the description (5/5 passing, included in the above).