Declare rustler explicitly so mdex_native force-builds

[timujinne]

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 declaration. 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_crm#25, BeamLabEU/phoenix_kit_warehouse#20, 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, with MDEX_NATIVE_BUILD=1 set under OTP 28 — the scenario this PR's rustler declaration targets. The declaration compiles and builds the native NIF cleanly. This module's config/test.exs already supports pointing at an existing database via PGDATABASE, so the integration half ran without any local workaround.

1123 tests, 3 failures (2 excluded).

All 3 predate this PR and are unrelated to it — this PR only adds {:rustler, ">= 0.0.0", optional: true} to mix.exs/mix.lock, and touches nothing in the failing modules. The failures are in EntityDataCreatedByTest and EntityDataCreatedByWithoutUsersTest, both of which assume a database with no pre-existing users. The shared test database this suite ran against carries one pre-seeded admin account — a property of the test environment, not of this module's code — so created_by falls back to that seeded user instead of the test's own fixture, and the "no users exist" scenario the second module exercises never actually holds here.

I read each failing assertion individually to confirm this; none of it is caused by this PR.