feat: set DM_BASE_PATH so the viz-app serves under its Ingress sub-path

[alanbchristie]

What

When a DataVisualisation is deployed, the Ingress exposes the viz-app at a
per-instance sub-path (/<instance-id>). The app was served from /, so its
assets, client-side routes and /api/... calls resolved at the origin root
— which the path-based Ingress does not route to the Pod — and the page
rendered blank.

squonk2-viz-app now supports a DM_BASE_PATH env var
(InformaticsMatters/squonk2-viz-app#7). This change has the operator set it.

Change

build_environment appends DM_BASE_PATH = "/<instance-id>" whenever an
instance id is known, and omits it otherwise (the app then serves at /, as
before). The Ingress is unchanged — it already routes the /<name> sub-path to
the Pod.

if instance_id:
    env.append({"name": "DM_BASE_PATH", "value": f"/{instance_id}"})

Tests (TDD — written first, seen red before the change)

• test_build_environment_sets_dm_base_path_from_instance_id → /instance-111
• test_build_environment_omits_dm_base_path_when_instance_id_absent → key absent

All 33 tests pass; black clean; pylint 10/10. (The pre-existing mypy-strict
complaint on the @kopf.on.create decorator is unrelated and present on main.)

Note

DM_BASE_PATH must match the URL prefix the Ingress serves (/<name>). This
relies on the CR name equalling the …/instance-id label value — confirmed as
the case for this deployment. Worth an end-to-end check on a real cluster once
the viz-app fix is released, since this is the first time both halves run
together.

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