Add source citation requirements to validation

[TommiLindfors]

Scope

Add validation rules that enforce source citation requirements: every publishable object must have non-empty source_ids, and each referenced source ID must correspond to an existing Source object in data/sources/. Add this as a step in the CI validation workflow.

Source links

• Internal: scripts/validate.py
• Clarvia source-first data model specification

Acceptance criteria

• Validation checks source_ids is non-empty for publishable objects
• Validation resolves source_ids references to actual Source files
• Clear error messages for missing or broken source references
• CI workflow updated to include this check
• Existing seed data passes validation
• Review status assigned

Trust level

maintainer-only