feat(api): add /structure/{type} handlers for all DPM artefact types

[antonio-olleros]

Wires REST endpoints for the 11 remaining artefact types alongside the existing category and release handlers, following SDMX-style URL grammar (wildcards, ~/+/* release tokens, pagination, allstubs detail).

Notable shape decisions:

• modules embed children=tables (full shape); tablegroups embed tables and child-group stubs, both release-versioned independently;
• contexts use virtual versioning driven by downstream property/item code drift;
• operations carry the full nested tree (versions → nodes → operand references → locations) inline;
• properties version through ItemCategory rows in the property meta-category and surface enumeration via PropertyCategory.

Every endpoint batch-loads its dependents; per-endpoint query-budget tests prevent N+1 regressions. _get_all_releases now sorts NULLs LAST so null-dated releases (e.g. EBA's 26.0.10) don't break the virtual- versioning iteration in _version_at_release. STRUCTURE = "structure" removed from ArtefactType — it was never a real artefact.

Summary

Checklist

• Code quality checks pass (ruff format, ruff check, mypy)
• Tests pass (pytest) with 100% branch coverage (coverage report --fail-under=100)
• Documentation updated (if applicable)

Impact / Risk

• Breaking changes? (public API / CLI / REST endpoints / Django models)
• Database schema or migration concerns?
• Notes for release/changelog?

Notes

[Copilot]

Pull request overview

Adds REST router wiring and tests for additional /api/v1/structure/{type} artefact endpoints, extending the structure API beyond the existing category/release handlers.

Changes:

• Adds registered handlers for table, tablegroup, module, framework, property, variable, context, operator, operation, datatype, and organisation.
• Adds references=children support and removes the non-artefact structure enum value.
• Adds broad endpoint, detail, references, empty-result, versioning, and query-budget regression tests.

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/dpmcore/server/routers/structure.py	Registers new structure handlers and updates artefact/reference enums.
tests/unit/server/test_structure_table.py	Covers table endpoint payloads, versions, variables, and references.
tests/unit/server/test_structure_tablegroup.py	Covers tablegroup hierarchy and children expansion.
tests/unit/server/test_structure_module.py	Covers module endpoint, table children, and query budget.
tests/unit/server/test_structure_framework.py	Covers framework endpoint and module children filtering.
tests/unit/server/test_structure_property.py	Covers property versions, enumerations, and references.
tests/unit/server/test_structure_variable.py	Covers variable versions, enumeration, keys, and references.
tests/unit/server/test_structure_context.py	Covers virtual context versioning and compositions.
tests/unit/server/test_structure_operator.py	Covers unowned operator endpoint and arguments.
tests/unit/server/test_structure_operation.py	Covers operation versions, nodes, references, and locations.
tests/unit/server/test_structure_datatype.py	Covers datatype hierarchy and children expansion.
tests/unit/server/test_structure_organisation.py	Covers organisation filtering and response shape.
tests/unit/server/test_structure_versioning_regression.py	Adds release-ordering and per-version enumeration regressions.