add key-level translation strings and jurisdiction content

[seanmalbert]

This PR addresses #6508

• Addresses the issue in full
• Addresses only certain aspects of the issue

Description

Adds the Phase 1 data model (content from the database): the key-level TranslationStrings table, the JurisdictionContent table, the SiteEnum and TranslationOrigin enums, and migration 66_key_level_translation_content.

• TranslationStrings: one row per (jurisdiction, language, site, key) with value, a source_hash (for staleness detection in later work), and a machine/human origin. The unique index is created NULLS NOT DISTINCT in the migration SQL (Prisma cannot express it in the schema) so base rows (null jurisdiction and site) and global-Partners rows (null jurisdiction) cannot be duplicated.
• JurisdictionContent: typed JSON documents (footer, faq, resources, disclaimers, contact) per (jurisdiction, language), FK to jurisdictions with ON DELETE CASCADE.
• Back-relations added on Jurisdictions.

The change is additive: nothing reads the new tables yet, and the legacy translations table is untouched. It is retired in a later ticket once the read services are switched over.

How Can This Be Tested/Reviewed?

1. Pull the branch and apply the migration: cd api && yarn prisma migrate deploy. It should apply 66_key_level_translation_content cleanly (requires Postgres 15+; Bloom runs 18).
2. Confirm the translation_strings unique index is NULLS NOT DISTINCT:

   SELECT indexdef FROM pg_indexes
   WHERE indexname = 'translation_strings_jurisdiction_id_language_site_key_key';
   -- ... (jurisdiction_id, language, site, key) NULLS NOT DISTINCT

3. Confirm duplicate null-keyed rows are rejected. Base rows (null jurisdiction and site):

   BEGIN;
   INSERT INTO translation_strings (language, key, value, updated_at) VALUES ('en','x','a',now());
   INSERT INTO translation_strings (language, key, value, updated_at) VALUES ('en','x','b',now()); -- expect unique violation
   ROLLBACK;

   And global-Partners rows (null jurisdiction, site = 'partners') the same way; the second insert should raise a unique-constraint violation.
4. yarn prisma migrate status should report the schema is up to date, and yarn prisma validate should pass.

Author Checklist:

• Added QA notes to the issue with applicable URLs
• Reviewed in a desktop view (N/A: backend schema/migration only)
• Reviewed in a mobile view (N/A: backend schema/migration only)
• Reviewed considering accessibility (N/A: no UI)
• Added tests covering the changes (no unit-test surface for a schema/migration change; the models are exercised by the services in Translation read/merge service and public read endpoints #6509+ and the integration/e2e in Integration and end-to-end tests #6521)
• Ran yarn generate:client and/or created a migration when required (created 66_key_level_translation_content)

Review Process:

• Read and understand the issue
• Ensure the author has added QA notes
• Review the code itself from a style point of view
• Pull the changes down locally and test that the acceptance criteria is met
• Either (1) explicitly ask a clarifying question, (2) request changes, or (3) approve the PR, even if there are very small remaining changes, if you don't need to re-review after the updates

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