feat(i18n): add locale query param to opportunity and suggestion GET endpoints | LLMO-5430

[Sihalnic-Alin]

Changes Made

Adds a ?locale= query parameter to all opportunity and suggestion GET endpoints so that AI-generated content can be returned in the user's selected language once audit workers begin storing translations alongside English content.

• OpportunityDto: toJSON(oppty, locale) reads data.i18n[locale] and promotes matching title/description values to the top-level response fields; strips data.i18n from the response so the shape stays stable
• SuggestionDto: toJSON(suggestion, view, opportunity, locale) merges data.i18n[locale] on top of the data object (covering title, rationale, description, expectedOutcome, actionItems, persona); strips data.i18n from the response
• OpportunitiesController: all three GET handlers (getAllForSite, getByStatus, getByID) extract locale from query params and pass it to the DTO
• SuggestionsController: all five GET handlers (getAllForOpportunity, getAllForOpportunityPaged, getByStatus, getByStatusPaged, getByID) extract locale and pass it to the DTO
• OpenAPI (parameters.yaml, site-opportunities.yaml): new locale query parameter defined with regex ^[a-z]{2}_[a-z]{2}$ and referenced on all GET opportunity/suggestion endpoints
• Tests: new test/dto/opportunity.test.js (8 cases) + 5 locale projection cases added to test/dto/suggestion.test.js

Storage contract for audit workers (Phase 2 — separate PR per worker):
Translations should be stored in data.i18n[locale] when creating/patching opportunities and suggestions:

// opportunity.data
{ "i18n": { "fr_fr": { "title": "...", "description": "..." } } }

// suggestion.data
{ "i18n": { "fr_fr": { "title": "...", "rationale": "...", "actionItems": [...] } } }

Missing or absent translations fall back to the original English values — fully non-breaking.

Companion FE PR: adobe/project-elmo-ui#2124 (localizes static UI strings; Phase 3 — passing ?locale= from the UI — is a follow-up once audit workers populate translations)

Related Issues

Relates to LLMO-5430

Testing the PR changes

1. Run unit tests: npm test — all 497 tests pass
2. Start the service locally (cp .env.example .env && npm start)
3. PATCH an opportunity to inject a test translation:

   curl -X PATCH \
     -H "x-api-key: api_key_for_user_requests" \
     -H "Content-Type: application/json" \
     -d '{"data": {"i18n": {"fr_fr": {"title": "Titre de test en français"}}}}' \
     "http://localhost:3002/sites/<siteId>/opportunities/<opportunityId>"

4. Fetch with ?locale=fr_fr and verify title returns the French value:

   curl -H "x-api-key: api_key_for_user_requests" \
     "http://localhost:3002/sites/<siteId>/opportunities/<opportunityId>?locale=fr_fr"

5. Fetch without ?locale and verify the original English title is returned (non-breaking)
6. Verify data.i18n is absent from both responses

Screenshots/Videos

No screenshots available.

Additional Notes

N/A