Schema checker: stac-validator wrapper (checks/schema.py)

[GeoSpark]

Wrap stac-validator to emit normative Findings. See DESIGN.md §6.

Scope

• backend/checks/schema.py: run stac-validator on the item; map schema errors to Finding(category=core|validation, confidence=normative, source="stac-validator"), with json_path where derivable.
• Cache/pin schemas for offline test runs where feasible.

Depends on

#2, #3

Test approach

TDD / integration. Known-good and known-bad fixtures; assert emitted findings. Guard any network-dependent schema fetch behind a cache or mark as integration.

Acceptance

• Valid item -> no schema findings.
• Broken item -> mapped Findings with paths.