Add institution record for Bureau de l'état civil and link from death certificate

[TommiLindfors]

Context

In PR #34, @HirenGajjar verified that the death certificate issuing authority is the Bureau de l'état civil (civil registrar's office) of the communal administration. The issuing_authority_id was left as null because no institution record existed yet. This issue closes that loop.

What needs to happen

1. Create a new source record

The death certificate Guichet page is a distinct source from the main death life event page. It should have its own source record.

File: data/sources/lu/guichet-death-certificate.yaml

Source URL: https://guichet.public.lu/en/citoyens/justice/certificats-copies-actes-casier-judicaire/certificats-copies-actes/acte-deces.html

Follow the pattern in data/sources/lu/guichet-death-life-event.yaml. Include:

• id: use the pattern source:lu:guichet:death-certificate
• title and title_fr
• description and description_fr
• source_type: government_portal
• Relevant fields from schemas/v0.1/source.schema.json

2. Create the institution record

File: data/institutions/lu/bureau-etat-civil.yaml

Read schemas/v0.1/institution.schema.json carefully before starting. Key fields to research:

• id: use the pattern institution:lu:bureau-etat-civil
• name, name_fr, name_de
• institution_type (check the schema enum)
• jurisdiction
• url (link to the relevant Guichet page)
• source_ids referencing the sources used

Note: The Bureau de l'état civil exists in every commune. This record represents the institution type, not a specific commune's office. Clarify this in the notes.

3. Update the death certificate document requirement

File: data/document_requirements/lu/death-certificate.yaml

• Set issuing_authority_id to the new institution ID (institution:lu:bureau-etat-civil)
• Add the new source record to source_ids
• Update provenance.updated_at and updated_by
• Update notes to reflect the completed link

Acceptance criteria

• python3 scripts/validate.py passes
• New source record follows existing source pattern
• Institution record is schema-compliant
• Death certificate references the new institution ID
• All source citations documented in PR body
• 3 files created/modified total

Difficulty

This is a multi-file task involving 1 new source, 1 new institution, and 1 update. It builds directly on the research done in PR #34.